Why did your son hit the other boy?  Was this addressed at the time?

I will offer advice/thoughts once I hear the above.

I wouldn't sit around your family's dinner and let your kid be hitting on my kid all day either.

Whether the little boy is biologically his blood or not, would you want another child to be mean to your  child? Like it or not, he is raising the child as his. 

I'm assuming you dealt with the hitting issue? I really don't see why your husband would overreact so much over it. He has a child himself. Kids that age tend to be assholes sometimes. They don't have the ability to properly process their feelings yet. They're impulsive because their brains aren't developed enough to NOT be. It gets much better around 5-6 years old. Are there other issues going on that would lead your husband to get so upset? Is this an ongoing issue and you're not addressing it? 

I mean, like I said, 4-year-olds do stupid stuff like that, but as long as you're consistently on it when it happens/when you find out about it, he should understand. Seems like there's more to the story or your husband is a hothead. 

Is he you husband or fiance?  Abandoning you over something a 4 yr old did is pretty sh*tty.  I get that he was mad that your kid was hitting his but that is the time to step in and discipline the kid who hit and the kid doing the taunting.  Running away only teaches the kids avoidance.  

With kids that close in age the both of you need to get a handle on this before it gets worse.  

What did you do when your son hit SS? Did you even try to discipline him?

If you did, and your FDH took off, he is a jerk and you should stay at your parents and move on and count your blessings that you didn't marry him yet. 

IF you did nothing- I can't blame him for getting mad. I would have been furious. If the roles were reversed and it was SS hitting your son, you would have been too. It really doesn't matter if SS is biologically his or not at this point. SS is being raised as your FDH's kid. You have to just put it out of your mind. Chances are that even if it was found out that he isn't his child, your FDH would still be responsible for him. That is how it works in family court. The best interest of the child is to keep him with a father, even if he isn't genetic unless the real bio dad is willing and able to step in.

I'm sorry, it's ridiculous for him to leave you there for any reason, like a child taking his toys and going home. If you guys are going to be family, he'd better get used to sibling squabbles including young children hitting each other.  He should have pulled you aside, asked you how you wanted to handle it, and if needed, let you know he'd like to leave because the kids aren't getting along.

Very emotionally immature.  I'm surprised at all the posters claiming it's just fine for him to do that.  How are you guys going to live together if you can't work out disagreements between your children?  If my DH ever left me at a party for ANY REASON, it would be a big issue for me.

He needs to grow up. If his son was being mistreated at any other party by a non-relative kid, fine, but you are either his wife or fiancee (you said both) and he needs to work it out like a grown man.

Are you sure you want to marry this guy? He's rather immature and you seem to have a real problem with his kid and the fact that it might not be his. Your resentment over this will just continue to grow.

Are you sure this is exactly how it went down?

Are you sure you didn't throw in a "you don't even know if he is your son" before he sped off?

The man is raising a kid he isn't 100% sure is biologically his, so either he is suffering from some serious guilt over not fully believing the kid is actually his OR you're really letting it show that you have a problem with the fact that SS, despite being LEGALLY your FDH's, may not be biologically his.

I just have a hard time belieiving someone stepping up to raise a kid they aren't sure is theirs would be that petty with kids fighting and hitting each other only. Something else had to have happened, or he's really bonked up in the head (in which case I don't understand the appeal for marriage).

I'm thinking the same thing. OP was with HER family and HER kid was hitting the other child and she's sure to remind us that the kid might not even be his.

I have a feeling there's ALOT more that went down. There's also ALOT of excuses as to why she's not at fault and why her child hitting isn't that big of a deal because hey he’s a hot head was being taunted by the other kid who OP clearly resents.

Personally OP you need to leave this relationship. What do you think will happen if you NEVER find out if the kid is SO’s or not. What if the child isn’t? What if the child is? None of those ensure any sort of future. Biology DOESN’T make a parent.

If you don’t want to leave you need to accept that the child’s biological relationship to your partners doesn’t matter. I’d also look into seeing a therapist as a couple because you’ve got a lot to work through.