Oh man sorry to hear that. You could see if he could special fit the cast for swimming maybe. Other than the pullups, pushups, possibly pistols, and a few others I would try it out and see.
When I was in Ft. Bragg on my Orthopedics rotation we had one of the Airborne guys who broke his ankle and was training for a triathalon. He did nothing but swimming in the cast. The surgeon said the guys rate of healing was astounding. He ran the Tri 2 weeks after having the cast removed. Pretty impressive.
"Permanence, perseverance and persistence in spite of all obstacles, discouragement, and impossibilities: It is this, that in all things distinguishes the strong soul from the weak”