When you work out and build lean body muscle, you gain weight because muscle weights more then fat. Your son exercises numerous hours for an 11 year old plus he plays sports on top of the exercise. I recommend you put the scale away and get a measuring tape. Just have him measure his chest, waist, and thighs. You want to see the chest / thighs build up and the waist go down. If he does not want to measure, then a good indication that he has weight loss (but the scale says differently) is that his clothes are loose fitting more then before. If that is not the case, then have his MD run tests and check him out.