Is it wrong for me not to want to go to my girlfriend's house because her son acts horrible?
You need to find another girlfriend
You're right, he does need to find another girlfriend. She and her son are a package, period. If he can't handle the son, he doesn't deserve the mother. If mom can't deal with her son, she needs to focus her energy there, and not on a boyfriend.
She needs to get a handle on the kids.