Sometimes the easiest answer is the right answer.
Talk to him and make sure he understands. You mentioned you have talked to him before, this time try a different approach. And never ever be rude and respond when you are angry (since you mentioned you snap at him). If you snap at people in a work environment, no one, and mark my words here, no one will take your side ever, no matter how much he is at fault.
Instead, do this,
- Setup a 1:1 with him; and say you need to talk.
- In the meeting, talk to him in a very detailed way how much him teasing you affects your day to day. He will say you guys can’t take it, just say yes your are right, and it affects me.
Before going to meeting, do prepare a bit. List down a few instances so that the conversation is a bit more directional. In the conversation, he sure will defend him self, but don’t challenge that. Don’t go too deep in one instance, just show him a pattern.
If you like your startup, don’t quit, learn to manage such things. These will be very crucial in your career.