My advice to u is to sit down and have a real convo with him. Don't try to talk to him when he's upset and needs space, cuz that'll just make things worse. Wait until he's calm and then talk to him about how u feel and what u want. Make sure he knows that u appreciate the things he did for u, but also make sure he knows that u don't want to argue about the tip every time. If he can't afford to leave 20%, that's cool. But if it's a big issue for u, then u gotta let him know.

Just be honest with him and communicate. And try to have a good time on ur bday, cuz that's what it's all about. Good luck bro!