the unfortunate reality is that the world isn't set up for kids to move out at 18. you still need an adult to do so many things (co-sign, for example) until you're older. plus you have no permanent address, everything gets fucked.
and please don't give him the generic bullshit about "parents want what's best for you." people say that and it completely shits on kids who know that their parents aren't doing what's best for them. kids who have to live through abuse in all forms every day. some parents abuse their kids. to a lesser extent, some parents think they know what's best, but their opinions don't fit their kid's needs.
every kid's situation is different. don't generalize.
my parents wanted what's best for me in school so they could brag about their son being at the top of a class of 500. i make a couple decisions they don't like, and boom i'm on my own trying to fight through college with no backup from them.