Jen - if you are looking for a 1 inch thick board you would want the green belt option. "1.75" does indeed mean 1.75 inches thick, but don't bethrown by the color gradings - it's not necessary for all brown belts to break the brown belt board, it's just a clever way to name them.

If I could make a suggestion: try to refrain from having your son do breaking at such a young age. Children's bones are in a state of development at that point and aren't adequately hardened to withstand that kind of punishment. If breaking is required for black belt, just have him wait for a few more years to achieve it. Black belt will also mean more to him when he is old enough to appreciate all the implications that come with having a black belt.

Actually I think they have it labeled incorrectly on here. The most common board size broken is 10 inches by 12 inches and one inch thick. The green board sold here has the same break tension of a 10"x12"x1" board. A white board has about a quarter of that tension. If he needs to break a 12"x12" I would recommend getting a couple. Like a brown and a black. It's a wee bit spendy, but you have to also remember that all breaks are not considered equal, especially at your sons age. Okiken108 is definitely right. Just because it might be safe for him to do elbow strikes or kicks on a tougher board (like the black one here) doesn't mean he should often be doing things like punches, for instance. For that, pick up an easier board so he can still practice breaking with those techniques safely.