from what I understand, you completed the picture before he even paid you? if so, I would advise against doing so in the future, with any commissioner. If a commissioner wants to see progress before they pay you, I think a good compromise would maybe be to only finish as much as a refined sketch before you get paid, or perhaps even require a partial deposit (30-50%) before you begin your work altogether. I think that completing an entire drawing or painting before you get any payment is, no offense, just asking to be taken advantage of :/

I'm really sorry that happened though... and thanks for the warning on this guy.