I would have to say that he is reacting to change in his life. How long have you been divorced? Is there another woman at his dad's house?
The reason I ask is because, I am the stepmom of a 17 year old and an 8 year old. They were 5 and 14 when I came into the picture and we had the same problem with the younger one. The biggest problem for us was that his mom let him get away with everything. We actually have rules that we expect to be followed. He has chores that need to be done daily and homework. After a weekend with his mom, who would just spoil him for a couple days, he would have to come home to reality. He didn't like it and would throw tantrums. I believe it is more of an adjustment period, and a stage that he will outgrow. Ask him about his time with dad. I am pretty sure that if he only has him for the weekend, they always do something extra special which means no chores, no responsibility for anything. What kid doesn't want that all the time. What they don't understand is that we have them 98% of the time and we are the one's that are teaching him what life is really about, responsibility and respect. I would say, just try to keep calm and in time it should all work it's self out. Keep teaching him what you know, keep the one on one time going on, and DON'T GIVE IN TO THE TANTRUMS! Once he realizes they don't get him anywhere they will mellow out!
Hope this helps a little...
R.