Hugs to you and your son. I am so very sorry that you are going through this.
Excellent suggestions here. With boys possibly more so than with girls, sports can seem all-too-important and for some boys at least, it seems difficult to separate self-esteem and athletic ability. This is completely unfair of course to boys with other talents, but it can be so hard to see outside of peer groups, especially when one is young.
I would agree with the suggestions above about an animal companion for exercise. A dog with an outstanding temperament (if he is not allergic, although there are breeds with lower allergens) can offer unconditional love and companionship, as well as serve as a motivator for exercise. Dog sports such as agility or obedience offer a sense of athleticism and teamwork that can build a sense of well-being and joy.
Please continue to let us know how your DS is doing.