After my hamstring lengthening, I used aquatherapy (therapy in a pool) and gained strength back very quickly. Even if you can't find a PT place with that, maybe you could try doing some exercise at the pool this summer with your son. Let's see, I did things like walking in place, kicking one leg out to the front, side, and back; floating on my stomach and trying to bring a leg out of the water, floating on my back and kicking my legs (like pedaling a bike)...and probably other stuff I can't remember. The water is good because it holds you up somewhat while adding resistance. And it's more fun than regular exercise!

I had to have someone with me in the pool, though.
What exactly do you mean by he can't walk with a walker? When I was in the pool they first made me stand in place to gain more balance, and then I walked around by myself a little. It was hard, but it did help my balance outside of the water...like I said, the water makes it easier because it holds you up.
HTH...good luck