Hi I've been diagnosed with lupus nephritis at the age of 15,
I was on 40mg prednisone per day, among others, I had the normal moon shaped face, eating alot and a steady weight gain.
However after my Dr decreased the dosage I gradually started losing all the extra flab.
So not to worry, keep him on his normal excercise and diet regime and after the prednisone starts decreasing he will notice a decrease in weight, and also, and i stand corrected, because he is in puberty stage the extra testosterone might help him even further.
All the best and hang in there. Remember he needs all the love and understanding he can get.