Confused

So my fiance and I each have kids from previous relationships and I am currently pregnant(34 weeks) with a son that belongs to my fiance and I. It seems like we r fighting alot more lately when it comes to our kids. We were raised so different and it makes things very hard when it comes to our kids. He just told me that his ex wife wants to know if he wants to do something together for their son for his birthday ? It really upsets me. Am I wrong ?

He also just told me that if the fighting about the kids doesn't stop then we r gonna be done as well. We NEVER fight in front of our kids....

When its just my kids everything is fine most of the time but as soon as his kids come around its an argument between us. He changes...he is still good to me and my kids its just he acts different when his kids r around. Help...what should I do ???