I just read that he was being treated for asthma. Have they checked his cortisol level? Low cortisol or adrenal insufficiency can cause fatigue aming other symptoms. It is a rare side effect of steroids, inhaled or oral, but it does happen. Doctors tend to blow it off as a possibility- it literally took years for one of my sons to be diagnosed. It can be debilitating and even life threatening, especially if it is not being treated. It can be diagnosed with a simple AM cortisol test.