AITA for cutting my sons hair?

So my son is 5 and in kindergarten and today is picture day. Just for a little background, his father and I are no longer together and we try to co-parent as best as we can. When we were together he was abusive in every way he could be. Emotionally, verbally and financially. I finally left when he threatened to kill me because I told him I was going to call the cops on him for throwing shit at me during a heated argument because I just wanted to leave the house after my car got stolen. I was frustrated and needed to blow off steam, but I digress. It was very toxic but just because he was a shit husband didn't mean he was a bad father so I never kept our boys from him. We settled a schedule and he sees them every weekend. As it is now I have majority custody and he pays $400 in child support for both of our children (when because I was on state aid they wanted $900+) so I tried working with him on something "fair". Ok so my ex-husband has a little native in him and likes his hair long, which is fine I always liked his hair long. Well he's kind of adopted that for our boys and I haven't had a problem about it until now. My ex absolutely refuses and is adamant we don't cut his hair. My oldest is in kindergarten and was getting called a girl by just about everyone (accidental and kids being mean) and he hates it! To the point he took scissors to his own hair and made parts of it uneven. So my boyfriend asked him if he wanted it cut and my son said yes! I was on the fence about doing it because I know it's going to be an issue for my ex and he's going to get pissed but my son has been so unhappy about it. Not only did he hate the name-calling but he hated taking care of it and hated dealing with knots. So we cut his hair. My son is over the moon that he looks like a boy. But I kinda back pedaled because I know this is going to be a fight between my ex and I. My boyfriend reassured me that my son is old enough to have a choice in the matter and I should fight for his happiness. I am feeling really torn right now. Scared of ex's reaction but I also desperately want my son to be happy which is absolutely is now.