I bet your SO would be singing a different tune if his kid were the one being excluded.

I have four daughters but my older three are 6, 7, and just turned 10. Kindergarten, 1st grade, and 4th grade. All my girls are beautiful and socialable but middle child is very beautiful and outgoing and my oldest child's friends invite her over with my 4th grade for sleepovers and playdates and my Kindergartener doesn't understand why she isn't invited especially since she is closer in age to my middle child and would cry and cry when she was left out. So we met another family in the neighborhood with a little girl and now my Kindergartener has playdates and sleepovers of her own and this solved the problem. I think ECHO has it dead on. It's not a step family blended family brother issue.

In fact the doctor told me that having three girls so close in age was going to be a problem and that alliances would be formed and one kid was going to be left out and then the alliances would switch and a different kid would be left out. She was dead on right. I think that is just what happens when you have three of the same sex close together in age.

I feel a little differently about this.
While I don't believe in "forcing" one child to play with another, this is family. We're not talking just a couple of "friends" hanging out, we're talking about two brothers and a "step-ish brother" that lives together. You said your two boys were like best friends before so it doesn't look like your DS8 had any valid reasons to stop playing with DS6 right? Except that oooh now there is another 8yr old boy in the house. Your SO doesn't want to force SS8 into playing with DS6, that you can't change. But make sure SS8 is not telling DS8 not to play with DS6...years ago when my girls were younger I overheard my spoiled ass niece telling one of my daughters NOT to play with her two sisters because SHE didn't want to play with them. I told her ass, family doesn't do that to each other, if you don't wanna play with them fine then go home, but you don't ever tell them to NOT play with their own sisters. She's such a little turd, I love her but now at 16 she isn't close to any family really, my daughters don't get along with her and none of the other cousins do either, they are always cordial with each other, they don't fight or anything they just don't have anything in common, she is still a brat at this age. My brother is to blame and he 100% admits he turned her into a monster.

8 years old is old enough to learn about compassion and family values and importance. Talk to your DS8 about how your DS6 feels, I bet you it will turn out better than you think. Unless your DS6 is a brat and DS8 is all too happy to be done with him, I bet he would be more than willing to at least spend 15 20 minutes a day to play video games, board games, etc. I'm not saying tell him to spend every waking with him, but a little quality time every day. You can also create a routine or game night WITH you and your boys. They will learn by example, spending time together is so valuable and time goes by so fast. Be creative, choose something that both sons will have fun doing (good tip is to find something you enjoy too, you are more likely to keep up the routine if you have fun too), and if your SO and his son wants to be a part of it, tell him "Sorry, right now I want to spend time with my boys only" and if he's a jerk about it just say "You can't force me to play with you if I don't want to!"

AND actually the idea of "I'm not going to force my son to play with someone if he doesn't want to" when we're talking about family members is like teaching the boy "I don't have to listen to you cuz you're not my mom...I don't have to talk to you if I don't want to...I don't have to put my dishes away if I don't want to". Like I don't get it we're not talking about a couple of kids in school, in the neighborhood, we're talking about family, we should always make time for family, and if your SO doesn't consider your situation a "family", well I don't know then. Screw him. But if you two have long term intentions, that does need to change. Would you two ever make routine weekend outings that included some kids and excluded others just because you wanted to?

From experience on routine family time I think it will keep you and your sons bond nice and solid. My older kids - 1 girl in middle school, 2 girls in high school, and 1 boy in college, chat it up daily, play online video games with each other, sometimes one just sits and watches, they go out to the smoothie place after school, things like that. My college son is the girls taxi driver on most school days cuz the girls schools are on his way to college, he never complains about it, I've never even told him that he HAD to do it, when I gave him my old car after he graduated high school, he is the one that offered it so I started giving him gas money too, but when he got a part time job about a year ago, he told me he didn't need me to give him gas money anymore. But that's part of what family does for each other. There is no feeling of being "forced", its just what you do.
Last night, when he came home from work he walked in, immediately sat down and started playing with BS3, talking in his "Rocky and Bullwinkle" impersonation, saying hello what are doing there, did you eat all your dinner, can I play too? Granted it was only about 10 minutes, but it was something, even though he had class and work. My BD15 even gives the bratty SS7 about 15-30 minutes every other week (he's here EOW) to video games with him, he asked her and she initially said no but we talked about tolerance and inclusion, and now she has fun with him. I told her though that she doesn't have to keep playing with him if he starts to get whiny, bratty or out of line, so she just warns him "if you act like that, I'm not going to play with you anymore". And it always works like a charm.

another AND, I don't agree with BS6 having to go out and find a friend, if he doesn't already know kids in the neighborhood from school, what would you send him out and he just wanders around the neighborhood looking for a friend, knocking on peoples doors asking if they had a 6 year old in there to play with, maybe cuz I live in a major city, I would not do that. When my bios were younger, on weekdays, there was no going out to play with friends or friends coming over anyways, they were home everyday after school/sports/music, doing homework, etc. And again, issue is not about BS8 having a new friend, the boy lives in the house, they are like family.

I don't know, I just feel sad for your BS6, I really do...he shouldn't feel so lonely and excluded in his own home.

Sorry so wordy, step hell is enough to deal with but when it starts affecting bio's or kids in general on such a negative level it makes me sad.

Your 8yo should be encouraged to be kind to his brother and encourage him to join in on occasion. But should not be forced to include him all the time. There really is a middle ground.

It's good to encourage your younger to establish some relationships of his own, as you seem to have done.

Promote family time - movies/tv? The three boys get to sit on the floor *together*. The adults sit on the furniture. Go for a hike - the five of you. Make pizza together. And so on and so forth.

Being mean isn't okay, but wanting to spend one-on-one time with a friend?