Unfortunately, he probably IS "outgrowing his legs". Cut back a bit on his feed- don't leave him enough to consume EVERYTHING he wants 24/7. I eliminated the problem of weak legs in my Cornish Cross birds by simply feeding them *nearly* as much as they wanted, but making sure the feeders were empty for about an hour a day. It takes some experimenting, but rest assured you won't starve him to death.
Feeding him some nice palatable weeds- lambs quarters, pigweed, or garden trimmings- will cut his calories a bit and keep him happy- and healthier.
I feed ALL my meat birds a vitamin- electrolyte supplement. It actually goes in their water. Most feed stores will carry something similar.
I've read that biotin is THE important nutrient in making sure their legs stay strong enough to carry them. The supplment I use (broiler booster from Murray McMurray) contains it.
I'm growing all heirloom breed turkeys this year, and they never "sit down" except when they're roosting at night. My Broad Breasted Whites and Bronzes over the last few years didn't much either- except for one who slipped a tendon as a young bird (he was an unexpected gift and I didn't have a pen ready- putting him on newspaper in a box was a mistake).
The heirloom breeds simply don't grow as fast as the "modern" types and don't seem to have the same problem with overeating and "outgrowing" their legs.
Hope this helps
Summerthyme