He needs to wait until he is completely done growing up , meaning height wise too.
Taking steriods at too young of an age, will cause premature closing of your growth plates. This means you will never grow as tall as you would have if you did not take steriods.
Actually the only steriod that has been proven not to cause this is anavar.
This steriod is actually the steriod of choice for children if they have to be persribed for most reasons.
I am not saying for him to take anavar, but if he were to take anything, that would be the one steriod I would suggest is fairly safe.
And the question he was concerned about was gaining alot of weight.
Well, anavar is known mostly to cause strength gains , and will also harden you up.
So it would be a good choice for him in my book if he is set on taking something.
He really needs to stay away from all the harsh steriods, like anadrol, tren, d-ball, and even winnie and deca.
Those all can have some serious side effects that will be terrible for someone that young to get, like premature hair loss, lowered sperm count, or even becoming sterile, kidney and liver trouble too.
also, they say that if you go on steriods too ealy in your training , you will usually miss out on about 20lbs of natural muscle growth that you will never be able to aquire once you have taken steriods.
Just my .02 cents!