I agree that something about the move must have been rough for your four-year-old. He may be reacting to a disruption in his routine, but perhaps there have been some important relationship--possibly a close relative, friend, or babysitter--that has ended suddenly and caused him to have a reaction that would be grief in a grown-up person.
But the loss of 5 pounds is a significant one for a four-year-old, and I would be concerned if he does not return to his usual self in a week or two. I would try to be as comforting and supportive of him as possible and try not to make a big deal about meals. You are probably correct that he is feeling powerless right now, and is trying to assert control over one area where he feels he has some control.
Spending more time with him doing pleasant activities he enjoys and reminding him in a friendly way how much you love him may help him feel more emotionally secure. If he does not seem on the road to genuine improvement in a week or two, I would consult a pediatrician.