Assuming it's not a learning disorder (and perhaps even if it is), I've found this:
http://www.gamespot.com/pc/action/mathblaster1insearchofspot/index.html to be an extraordinary asset in a similar situation. A certain amount of play was required (say, half and hour each night) and overseen by parents. Though there were still complaints, it made the process more bearable for all parties involved, and proved to be extremely effective, too. Now, obviously with a release date in the mid-nineties it is probably no longer available, and should you manage to find it may be no good on your current system. However, an analogous (and slightly newer) game may provide some stimulation for your son and come to be, if not enjoyed, at least tolerated as a tool for learning math. Not to mention that, if you play your cards right, you should be able to adjust the level of difficulty - a challenge he may find invigorating
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*[Edit] I would like to mention that I agree with Cathy; teaching him memorization shortcuts - or, even better, encouraging him to come up with his own - is another effective way to help him. I'd also say that if he can do more difficult problems, encourage that. It may turn out that all he needs in some motivation, in which case I'd suggest speaking to his teacher about having him do a more advanced curriculum in math.
