I completely agree with Catmoves, and I appreciate that there's a lot of research and study shown, as well! Don't get that too often ;)
I would also point out, other than the option of possible depression, that he IS 15, and this is a very difficult, and very pivotal part of human life. He's changing, coming into maturity, and though I'm sure it's easier to look at him as your "baby brother," he isn't a baby anymore. His attempt at a very low-grade drug (which is, in fact, safer than alcohol according to a LOT of statistics) may simply be an attempt at trying something new, something that's available, perhaps that his friends are trying, and something that is seen as "counter-culture," as it is currently illegal. This happens with many adolescents, though they may not try drugs, they may try other things that would be seen as "bad" or "wrong." It's part of development to push boundaries and try new things.
I also think that his unhappiness in his current environment sounds like the larger, and more pressing issue. Sure, he got a scholarship, and that's wonderful, but if he isn't happy there, then perhaps it's not something that is correct for him! If he's not feeling that it's worth trying, putting in the effort, and if he's really unhappy, perhaps some other options should be explored. Even if those other options turn out to be some other things he can focus on while he gets through school, such as getting into some different kinds of exercise, exploring different art, or even learning something new and radically different to what "seems useful," he may find that this new focus is what he needs to feel better.
Does he have any friends in this new school? Has he made connections? Is he physically active? Are his classes things that he would normally be interested in? Are they difficult? All of these factors, and yet more, are things that could help or hinder him in feeling fulfilled.
I also have very many friends who smoke pot, and being that I live in one of the most pot-heavy places around, we get to see a pretty large variety of use. From your full-on wake-and-bake-and-bake-and-bake stoners, to your occasional "use for when you're in crazy pain or stress" users, I see, and have seen, it all. Very few people around here use other kinds of drugs, and very few of them use them the same as they would use pot.
And, frankly, I'd rather see my future children getting curious about pot than alcohol... but, I suppose that's just me.