How do I know when my son's behavior is truly related to his Pdd?

My 7 yr old son, who was diagnosed as PDD-NOS at age 4, is driving my fiance and I crazy with his behavior! A year and a half ago, the school psychologist told me that her tests showed that my son was GDD, not PDD. I understand that there is a difference to these 2 disorders, and his teacher and I both agree he shows more PDD than GDD. However, I do not understand why his behavior is so different for me, than it is for anyone else? When my son stays with a friend, or even with my fiance, he is good as gold, does whatever he is supposed to, without an argument. As soon as I get home, he starts acting out, saying "No", demanding his own way, and even resorting to screaming and stomping. There are even times he has made it clear that he wants to hit or scratch whoever is telling him no. He refuses to eat dinner, unless it is something he wants, like pizza, or grilled cheese. However, his dad says he eats just fine at his house. I have tried everything I can think of, time outs, losing privileges (like the computer, or tv), but to no avail. I have even tried the reward system, but it goes out the window when he realizes he cannot have the reward right this minute. I am at a loss as to whether this behavior is related to the PDD or GDD, or if he is just acting out, perhaps because I am the working parent in the home. Any suggestions would be greatly appreciated!!