There's a lot of good (and some bad) info out there in this thread already, so I'll try not to make this post too redundant. Let's just say I've dabbled in pot and other things in the past, through the latter part of high school and college, though I had hard lines with certain things (opioids for instance) that were never, ever crossed. The OP's nephew actually doesn't sound entirely dissimilar to myself at that age (minus the depression, I was a pretty happy-go-lucky kid for the most part). I had my E36 M3 together pretty well back then and I'd like to think I've turned out pretty well today.
First off, I saw "wax" mentioned somewhere up the thread and I don't think anyone has explained that one yet. Wax is basically a refined and more concentrated form of THC, sometimes also referred to as hash oil. If my room mate's little brother is any indication, it seems to be all the rage among the kids these days, though I don't remember seeing it much at all even just a few years back when I was still in college.
I've tried LSD on more than one occasion and I'll say that I truly consider it a positive life experience. I think it has permanently and positively altered the way I think about and perceive certain things, such as, for example, one's sense of self and significance in the universe. It is also how I learned what Coriolis force is, but that's another story. It is very difficult to explain this to someone who is not familiar with the drug, because the popular perception is that it basically creates a sort of carnival of silly hallucinations, but to the intellectual mind it can also be a deeply thought provoking experience.
That being said, a few words of caution. One, you must be in a good mental state going into the experience, as it will tend to accentuate any underlying feelings and emotions, for better or for worse. If your nephew is struggling with depression, LSD is probably a bad idea at this point in his life. Two, while the real stuff is still out there (and you will know it when you find it), there has also been a massive proliferation of other hallucinogens that are not as well known and come from god knows where. Sometimes they get passed off as LSD, sometimes they go by heir own name, usually an alphanumeric thing like 2CE or the like. Avoid these. Three, on a similar note, don't ever be the guinea pig. Let some other poor schmuck who bought from the same guy try his first, that way you know a) whether it will kill you and b) whether its any good. Four, LSD is not an all the time thing, if you're dropping acid every weekend you will fry your brain and become a complete space cadet in short order. Seen it happen to people, they start losing touch with reality. This is a very powerful mind altering substance and one which must be treated with respect.
I think the most important thing here is to ascertain why he is interested in these things. Different people with different personalities/issues/interests/ect I think are drawn to different types of drugs as a result of the above, as they are looking to either accentuate or hide from certain traits. Put another way, I think everyone has a "drug of choice" based on personality traits.
For instance, someone who is gregarious and outgoing might prefer coke or molly as it enables them to be up all night and be social. Similarly, someone who is more reserved might also have an affinity for those drugs also, as they would tend to counteract their natural tendencies toward social apprehension in a situation where that might inhibit them, for instance a college party. We'll call these people the "partyers." Someone who is intellectually curious, but perhaps burnt out with academics or isn't challenged enough, might have an interest in hallucinogenics as a new way to stimulate and explore the mind. Looking back, I think this was kind of my mentality and sounds like your nephew might fall into this group as well. We'll call them the "experimenters." I think the most dangerous group is those who are just looking to "get away from it all." These are the people unhappy with themselves, who maybe have some serious underlying issues or a bad family situation or something, and they seem to be drawn towards opiates, which allow them to detach from that reality. Let's call them the "escapists."
That's more than enough arm chair psychology from me, though. The point in all that rambling being that if there are serious underlying issues, those are what tend to exacerbate the drug use and lead to real problems, particularly if there is a family history of addiction. If he has his head on straight, his E36 M3 together, and has real goals and ambitions in life, more than likely he's going to be OK, outside of the risks already discussed in this thread.