I agree with Cindy J - I have two sons - a 13 year old and a 15 year old. They are very similar - very bright and they both get 98-99 percentiles on standardized tests - my oldest got a 4 (out of 5) on the Biology AP last May (he just finished 9th grade). Yet, they are very, very happy with their B's along with some A's here and there. You have to keep in mind that the grades are not yours and do not reflect on you (and that may be the hardest part of this issue - it was for me since I always made straight A's in school). The grades are your sons and in my experience - and as you have found - bribery doesn't even work. I know my kids could get straight A's, but there view is that the kids that do get the straight A's work 3x as long as them. So, they've done the cost-benefit analysis and decided it's not worth the extra effort.
Also, is your son happy? When I spoke to my younger son's science teacher at the end of the last school year, her view was that as long as kids this age are happy and are doing their work for the most part, then we parents should be happy.
Also, for the parents that feel that you should control the situation, check all their work, etc., while that works for a while, there will be a point where you can't do all of it. Also, with my kids, when they do ask for help, I don't always get the assignments! In the end, too much help will hurt them as their grades will drop in H.S, rather than go up and they will not have learned study skills on their own (we all have our own learning and organizational styles). My son's counselor says that it's far more important for colleges to see an upward trend than a downward trend. My older son has started off his sophomore year with a little more focus, so I'm hoping the trend is upward. But, again, B's are not bad grades and grades are not at all what colleges look at. You can also find lots of stories of folks that weren't successful in school that have been very successful in life -Owen Wilson, for example, got kicked out of St. Mark's for cheating on a math test. There's lots of other examples like this, especially for math-smart kids.