Is the male you have worth anywhere near the one you lost?
Would you even want to keep his?
If you want it, ask that he give you a receipt for the snake with a value on it.
If it is less, you could take him to small claims court for the difference.
I would send a bill asap, adding a reasonable "per-week/day" charge for all future care of his snake. This should help motivate this person to deal with your agreement.
(edit/add) I dont believe you can bill for past care that's not part of your contract.