My 10 year old son just diagnosed with hand foot mouth disease. He is generally feeling fine. He still play sports with his father twice a day. Okay?

Sure, but... Hand/foot/mouth is a viral illness usually caused by a virus called a coxsackie virus. Usually we see this in our part of the country in spring/early summer, but we too have seen some cases in our office the past few weeks. Your body will fight this off on its own, so if he's feeling ok, he can be as active as he feels ok for. He is fighting an illness, though, so insure plenty of rest and fluids.
Fluids/rest. During viral illnesses, I would suggest drinking fluids, monitoring his temperature, and resting more than usual. If he feels well enough, then exercise in moderation should be fine. The illness usually lasts about 7 to 10 days. I am glad that he feels well.