Regardless of how well this particular case works, I'm in favor of people being able to modify their bodies as they see fit. If he wants to become more muscular, morbidly obese, or ten feet tall, that's his right. Unfortunately, I could see things like this becoming illegal solely because they help people play sports too well.
I think the ethics on this wander close to the ethics of personal drug consumption, or at least smoking. Sure, it's his body and he should have personal autonomy, but there's a possibility (maybe high, maybe low, I don't know) that he could end up sick and reliant on others (medical professionals, family). I know its slightly different in the US what with the lack of socialised medicine, but it seems reasonable for s…
He could also get that way from driving a car, being thrown from a horse, or any number of activities that are a personal choice.
Maybe a bunch of people try this and end up with terrible side effects. Researchers will still be interested in the results if they know what a person did to themself. And then maybe some of them will have good results and then we'll all know. It's a mixed bag and probably a very tiny percent of the population. I say let em go for it.