You can gym hop at all the
DVC resorts. To me the best gyms are
saratoga springs and the beach club's. Have your son check them out they
both have steam rooms (saratoga has it in a separate section and they charge a small fee, he would have to ask) this is perfect for the winter rainy days(Summer?????). Beach clubs steam room (and hot tub) is in the gym.
Both places have access to running trails.
Have a great time.