I don't know if you have heard of Dr. David Amen but he would advise that you not react to the sulking and pay extra attention to him when he is not sulking.
Telling him to stop are getting mad at him will not help. Most likely it is a behavior that he used with his mom as a guilt trip and he probably got what he wanted that way. By not paying attention to his sulking you will not be feeding this attention seeking behavior and by paying extra attention when he is not sulking will reinforce his need for attention in a positive way.
Two really good relationship sources are Dr. David Amen at amenclinics.com, and Christian Carter.