I used to teach high school math and science - and I think for most kids, year round school would be very helpful. Most parents don't bother to encourage their kids, or even check to see what they are learning. So I always spent the first month of school reviewing the things they didn't remember after taking the entire summer off. It was an awful waste of class time, if you ask me.
On the other hand, my son (and a few others that I know) spend the summers learning advanced material, and actually working out their brains more than they do during the regular school year. I think it would be more of a hindrance to him to take away his summer vacation.
Perhaps it would be best to have year round school, but make sure that the classes are appropriately leveled so that every student gets a challenge in class. There has been a trend over the past few years to do away with levelling classes to spare people's feelings. In my opinion, this only makes those who are struggling feel stupid and bores the advanced students to tears. No one benefits from it.