I do not practice in your state.
Are you sure both names are on the title? They may be on the note, but might not be on the title. If your's is the only name on the title, you could be entitled to possession of the vehicle and you could sell it. You should consult an attorney in your state.
He agreed to pay $200 of the payment for how long and with what in return? If he doesn't pay you can look to small claims court to enforce a money settlement type judgment. If he is not paying and you are not paying, therre are credit damages that you each will be liable to the other.
What benefit or use of the car was he to receive??? or was this a gift to you on his part?
I dont practice in your state but if you both signed the loan, you will have to refinance the vehicle and "buy him out" to get his name off. You will then have to sue him in small claims court to get your money back from him. You should ask for all of the money he agreed to pay, your loan costs, and your out of pocket costs. You should get together any documents showing that he agreed to pay for the vehicle. If his name is on the title, he would have an equal right to the vehicle and that is a concern for you.