I hope you'll read a terrific book by Faber and Mazlish, How to Talk So Kids Will Listen, and Listen So Kids Will Talk. (You can read part of this really practical guide to communicating with your kids here: http://www.amazon.com/How-Talk-Kids-Will-Listen/dp/038081...) You'll get useful and parent-tested guidance on how to help your son communicate his emotional quandary and participate in finding his own solutions. I use this approach with my grandson, and am often surprised and delighted with how brilliant and original kids can be.
I don't know how long you've been divorced, but for some kids the strain of separate households becomes more difficult to deal with over time, rather than more routine. Your son may have really conflicted feelings that his dad is not helping, especially if he indulges him with gifts. That could be really confounding for a child who's trying to figure out how people, including himself, are supposed to work. The book above may help you help him sort through his feelings.
Another book that's also helpful in a similar manner is Raising an Emotionally Intelligent Child by John Gottman. Studies show that children whose parents practice "emotion coaching" are more physically healthy and emotionally resilient, less affected by stress, perform better academically, have better relationships, and are less likely to develop behavior problems. This includes children whose families split up.
Another thought I have is that at his dad's, he may be exposed to foods, additives, and/or air-borne pollutants that upset his chemistry. I have severe chemical sensitivities, and exposures can give me both physical and mental symptoms. In group testing situations, I would watch calm kids turn weepy, or hysterical, or angry and stubborn, mere minutes after diluted substances were placed under their tongues.
