When meeting the parents, dress to impress - don't over do it, don't wear anything skimpy or nutty. Be polite. If they seem off putting - don't take it personally, they don't really know you. Be yourself. Be friendly. Find something that you like about the mother or her home.
Also, take a good long look at the home and people. Are the parents kind to one another? Or are they mean to one another? Does the home feel comfortable, or does it turn you off with a bad vibe? Remember, they are the ones who raised him - he is a product of his upbringing.
Remember, they are not just judging you but you are also judging them!