My advice would be to pack very carefully and light. Let your son carry at least some of the gear, at a minimum his own clothes, sleeping bag, and sleeping pad. That can easily be less than 10 pounds. Let him feel he is pulling his weight. Adjust the pace and daily mileage as necessary to accomodate that.

You could keep your load light enough that you wouldn't have to even consider the maximum capacity of the BoB and that is what I recommend doing. When it comes right down to it you really need surprisingly little. I carried way too much on my first long tour (TA) including quite a bit of community gear and still wasn't at the capacity of a BoB.

That said, if you must load yourself down, I have seen folks carrying way more than the rated capacity in a BoB. I wouldn't because I have found touring more and more pleasant as I have shed more and more unnecessary stuff, but you can if you really want to.