Metabolic Myopathy - elevated CPK and Aldolose

My 17 year old son has preliminary diagnosis from rheumatologist of metabolic myopathy involving energy transfer issues in the cells. He is a big  kid - 6 feet 210 #  He is an athlete and this process has been going on for at least a year as he is definitely weaker than he was a year ago.  The doctor indicated that supplements may help - (phone call) without specifying which ones.  My question is this.  Will he ever be able to rebuild any of the muscle that is lost? Will over-doing it cause irreversible muscle destruction?  He was a D1 candidate for a baseball scholarship, but the rigors of workouts seem impposible at this point.  He is starting his senior season of baseball now.  He was also an elite hockey player but was not able to play hockey this year.  He had a pneumomediastinum a year ago and was on massive doses of prednisone for over a month as he also has asthma and was having flare ups.  Could this be related?