For what reason is my son gaining weight?

Weight gain: You son can gain weight by 3 methods. He could be gaining weight to increased fat production due to ingesting too many calories. He could also gain weight due to muscle (m) formation due to his exercising and increasing his m mass. He could also be forming edema, water, that would increase his weight. A pound (p) of water weighs the same as a p of fat or muscle. Which do you think is the cause?