Uh, when you say that you've taken steroids since you were 15, did you train aswell? I'm quite sure that your health improves overall quite much if you start to train with or without steriods... Training improves your asthma, gives you stronger bones combined with extra calcium.
When you're young (15 years old) you've got plenty of hormones in your body. Do you really want to take the chance to stop growing in length aswell if you're not fully grown? AAS can cause premature closure of the growth plate in long bones for anybody who's not fully grown.
Steriods also has side effects like:
Boldness
Acne/oily skin
Smaller testicles
Possible liver damage under cycles (more liver enzymes)
Heart diseases after long term (heart works overtime causing premature coronary disease)
Bigger breasts
Illegal
Unprovoked aggression
In the end, it does have some positive effects but also quite a few side effects. There's people using steriods for medical treatment but people abusing it usually takes 10 or 100 times more.
He could easily gain a fair amount of muscles by eating right and training well. You don't need to take steriods.