I would recommend getting out your measuring tape (or buying one if you don't already have one... they're cheap and worth owning) and taking his exact measurements... waist, hips, inseam... while you're at it, might as well do the shirt measurements too.
Then get on the websites for a few stores with consistent sizing (I'd do Oldnavy because they have their own brand so you don't have to worry that the sizing is different from one pair to another) and check his measurements against their size chart. That will be a lot easier than all of this guessing and checking.
It DOES sound like you need to be doing the "husky" sizes at this point if they regular sizes in the correct length are too tight for his waist, so as a matter of style, probably stick with those.
You might also want to stick with shorts/pants that are sized as S, M, L etc, rather than specific number sizes, because they tend to accommodate a wider range of sizes at each size than the number sizes which are very narrow (ie M might be sizes 8-10... which could probably fit your boy comfortably without looking silly). Target often has these.
Hope this helps.
T.