Your Ex sounds useless.
I think it's insensitive for SM to buy stuff fir her kids IN FRONT of your son. That's just mean. She had enough other tine to do that.
Reducing the time he sees your son, does sometimes happen in these situations. It could be because the SM doesn't like your son coming over and prefers it to just be their family unit.
A lot of stepmothers dread the stepchildren coming over. It interfers with their family set up.
The real problem here is your Ex, because he let's this happen. He doesn't care enough about your son and he's a crap dad to him.
Be probably isn't father of the year to his younger kids either.
I agree with the poster who said their life probably isn't as great as you think. Even if he's not happy, he's going to be broke paying child support for 3 kids. He will also not want people to think he's the common denominator in 2 failed relationships.
If his wife isn't happy, she knows that he'll probably get with another woman, who will resent her kids, the way she probably resents yours, just for existing and her not being able to have her own nuclear family, without the constant reminder of her husband's past.
This is a reason that many SMs I've spoken with don't leave the relationship.
Continue to provide stability for your son. If your son expresses not wanting to sleep over at dad's house, don't go down the road of asking for more child support.
I'd bet if you told your ex that DS didn’t want to sleep over anymore, he wouldn't care. It would make his life easier.