How well do you know him? If he'* really pushy about wanting the car now, as soon as you start, and won't wait...I'd be a bit worried he'* trying to get away with it. Does he really need it now...does he have another car he can drive?
I'd write up some contract showing how much he owes you for the car..etc.. Maybe try and get a down payment. But I'd at least do that, and get his phone #, etc...so you can keep tabs on him. If he lives near you and ends up trying no to pay you...you have a contract and small claims court would be an easy win for you.
But obviously that'* also a PITA to go to court.