TubeNet

no offense intended here, but how about trying a euph (we´d use a "Tenorhorn") instead of the big un? It probably will have the same mass and height ratio to your kid as a full sized tuba has in comparison to grown ups.
I guess THE main factor of kids playing musical instruments is having fun with it. That just may include being capable of handling the darned thing.
A beginning student of mine is 12. He uses a really small, lightweight F-Tuba and barely is capable of holding it. If he had to play even on my MW46, I would have recommended Tenorhorn as a start and have him switch to the good stuff a couple of years later.

I,too, would second the suggestion to have him start on euphonium and not tuba. Failing that, try a "tuba tamer", made I believe, by Wenger. I saw one on e-bay at the beginning of the week--maybe it's still there? This will hold the tuba entirely by itself--all your son has to do is put his mouth and hands on it and go!