offer him a few bucks, tell him you want the snake, this way he does not have to worry about it anymore.
ETA: seriously if someone else dropped the snake off to you, i would have prolly assumed it was "Their's", let him come back, and complain, explain to him someone else gave you the snake and that he should take it up with them, not you. i seriously doubt by the sounds of it he is gonna make any stink over it. or charge him a fee for the care you put into it if he really wants it back. include the hourly rate on that bill. Especially if you deem it is needed for the animal to see a vet, include the vet bill as well, make sure to explain to the vet what the situation is just in case. I am sure the vet would be more than willing to talk to the cops or a judge over the condition the animal was in and how it is doing now in your care.