I was going to say exactly what ultramarine said about reading - share your own joy of reading, rather than bribing him.
My dd was in a math pullout in 1st grade and suddenly started saying she didn't like math. It was such an abrupt change for her I feel like *something* must have happened at school, but we'll never know. She says it in a matter of fact way, and we acknowledge (humor her?) in a matter of fact way.
Now 12 yo, she has repeated this mantra from time to time over the years, even though it is at odds with her math performance in school, her elegant explanations of math concepts to her peers (as reported by teachers) her reading (big fan of Murderous Maths books) and her spontaneously bringing up math topics for family discussion.
After breezing easily through honors algebra this year, she finally backed off slightly by saying, "well, its not so much that I don't like math, but I don't like math homework."
Finally, I am beginning to understand.... she has generally mastered the art of maximizing her grades with minimal effort on her part. But math is the one subject where she has been assigned *daily* homework, and has been for several years. It is sometimes long and tedious, and there is no getting around it. Sometimes it is even challenging!! Being challenged is not terribly familiar to her, and annoying, and math is the place she is most likely to encounter it.
My advice would be to support his academic pursuits in the most fun, upbeat ways possible. Read to him, make math games out of every day activities, etc., but don't let on that it is something you care about. Help him rediscover the joys of learning. (My dd's twin brother had, from a very early age, a very sensitive detector for parental desires, and his instincts told him to resist at all costs. Potty training was brutal with ds (and a breeze with dd)! He'd be totally on to any efforts to coerce him with bribes, etc., and not keen on rational arguments to boot. But fun almost always worked. )
