I agree with this.
Good nutrition is important and lack of stress, since children who don't get good nutrition or are severely stressed can end up a lot shorter. But that's probably not relevant here. I'm guessing that none is getting excellent nutrition and he does not sound like he has a stressful life. :)
But stretching, bicycling, swimming? I would be amazed...
It is true that bone is constantly being remodeled through the interaction of two kinds of cells--osteoclasts, which break down bone, and osteoblasts, which build it up. Even adults have this going on all the time, so you can change the shape of bone by changing the way you use your muscles. Running and jumping will make your bones denser. (Bicycling, oddly, predisposes people to bone thinning, probably because of the lack of compression on any bones.) MAYBE if you spent an hour on a torture rack every day and didn't manage to destroy your joints, you might get longer bones. I don't know. I don't see it!