First of all avoid all his attempts at 'gaslighting' ie making you feel as if you've brought on these problems. You haven't, these issues were around before you came on the scene and are not of your making.
You could try counselling- it works for some couples, and you need to get your relationship back on track.
You also need to disengage- stop reminding him that he needs to leave with SS, try to stand back from BM's attempts to drive a wedge between you. DH is responsible for that child and not you.
I know how frustrating this can be- I tried to point his 'jumping' to DH soon after we met, thinking the light might come on and he would see things from my perspective. He didn't and I don't think he ever will. OSD has driven the wedge between us she always sought, I don't think it will heal.
Work on your self-esteem- try to be busy with your own stuff whenever these incidents occur and if DH talks about it pretend you didn't notice 'cos you were too busy watching Star Trek, or whatever...
Stick around- you will receive lots of good advice hear from people who know exactly how it feels.

everything that fairyo said and also, stop loaning him money. Let him suffer the full consequences of his choices, whether it is being late, not having money etc.

Unless therapy can help, I am not sure your marriage is going to make it. Do you think he is interested in getting back with BM?

I mean the as kindly as possible: stop nagging him about his kid.

So what if BM orchestrates a half day with your DH? Do you think he'll cheat? Run off with her? Confess his undying love?

No. She'll flirt and flirt and flirt, and if he's a good husband, he'll look at her like she has two heads. He'll redirect conversations back to the kid or small talk. Or he'll start talking to SS and ignore her completely.

Really, he's only torturing and hurting himself when he doesn't stand up to BM in these instances. If he's late dropping SS off, he'll get the earful from BM. If he doesn't schedule his own parent-teacher conference, he'll have to deal with whatever date/time BM gives him. Nagging him about it isn't going to do anything except frustrate the both of you.

So, stop. If he complains, offer an "I'm sorry, that sucks" and change the topic. The less you involve yourself and try to control how he handles this, the less he will involve you. Right now, he has two women in his life telling him how he is a horrible father for not doing X, Y, and Z. That's not your job or responsibility, so let it go.

Why are you parenting your husband so much? The things you were complaining about really have nothing to do with you.

If the time BM schedules for conferences and or a medical appointment don't work for your husband, then he will speak up to her or suffer the consequences. It doesn't affect you. Or shouldn't.

If BM gets mad because he's late dropping off the child, how does that affect you? She can deal with it with him or she wont. Maybe, they're both fine with it.

Stop reminding him of anything to do with his child, stop paying for his crap, if he wants to be an as* then let him shi* on himself when he can't pay for his own bills. Tell him that you expect to be paid back $___ by December 1st and once he pays you stop covering for a grown man. Once you stop paying for his stuff then you won't need to worry what he spends his money on.

I came to terms with the same issue you're having by realizing that there is an "us" and then there is a "them". "Us" is my SO and I, "them" is SO and his children. We are not 2 teams pitted against each other but there is a difference, I will never be part of "them" and I am cool as a cucumber about that because if I wanted to be in a club with kids I would have had my own. Almost as soon I figured out there is an US and there is a THEM I accepted it and moved on, having no expectations helps as well because then you are never disappointed.

No more comments to DH about how BM should be treating him, or how he should be treating BM. Stay out of things unless you are directly involved. If you have the kid eowe then there is plenty of time for “us.” Stop focusing on the time you don’t have together and plan for the time you do. Address finances directly with DH to ensure you are each contributing your fair share to the household- resentment over money is a huge relationship killer, especially with stepfamilies.

Good grief, he needs to pull up his diaper and stop acting like a snively toddler face. DH and I are married and obviously live in the same home and he has NEVER been to a Dr.'s apt with our BS7 (other than his very first check up at 1 week) and he has NEVER been to a teacher conference. I am perfectly competent and can relay to him any necessary information received. why in the world does he think he "can't be a father" because he didn't go with BM to a Dr. apt or a teacher conference???

Effective immediately, I would stop "reminding" him about crap (what time he should leave, etc) and let him deal with the BM fall out. I would also work out a monthly bill pay plan where your bills are split directly down the middle and I would not contribute one fracking cent extra.

Does BM give him a hard time about being late on Monday nights? If not, then why are you saying anything? If you are nagging him about something BM overlooks, then I can understand why he reacts the he does.

I agree with all of the above...stop being in charge of keeping track of things or reminding him of things for his child.

Your DH is behaving like a child.

I used to remind my DH of things like conferences or school events or the importance of being on time. He would reward me with some of the same behavior. I finally told him, I don't really care what you do, but I expect you to keep BM's drama out of my house, so I expect you to do what you need to do to make that happen.

My DH also has a tendency to act like a child when his kids are around. I think it has to do with his own guilt, shame, and sadness over his divorce. That's all fine, but he needs to deal with that on his own and not blame me. He would periodically snap at me because he "thought" I was saying something mean to his kids, although when we discussed whatever it was later, after I got mad at him, he would say "oh, I agree with that." Yeah, well, then don't jump down my throat because you think I'm saying something that I'm not and especially don't do that in front of your kids.

Stop lending him money for his son, keep your money for yourself, he works for a salary he can get by on it.

Now he exploded cause he knows you are spot on with your observation and he knows BM still have his balls, thus he's not telling her NO stuff it..

He's just to proud to acknowledge it, in the future do not remind him of time, he's a grown ass man and can keep his own time, if he complains cause BM is late... pretend to listen and reply with ,,, mmmmm then change the subject.

Simply tell him there's 2 people in the relationship not 3, you are not a sister wife, if he wants to jump for BM then you should split and he should take her back,,, walk away

DH stopped joint Parent Teacher conferences with BM right away because BM used the time to complain to teacher about DH instead of focus on the skids.