if no contract was in place, neither side will be able to come after the other with a strong case. If he really is an illegal immigrant, he's not likely to use legal means to collect his money.
The correct path then becomes a moral and risk question. What's the right amount to pay based on what he provided for you? Do you want an illegal immigrant with nothing to do holding a grudge? You might consider paying the $500 just to make the issue go away.