Geez, Fran, this is quite the dilemma. It seems to me like you are already doing everything humanly possible. You can't pick him up and put him on a treadmill after all.
You are providing a healthy diet and the means to get exercise. As hard as it is, it is ultimately up to him from that point.
And yet, your mommy heart will break if something happens to him.
I don't think there is an easy solution to this one.
Sending hugs.
~Kathy