This kid merits being saved from his destructive course, and requires intervention. You've drawn a lot of conclusions without any evidence, and you're a bit dickish about it, and that's fine, but sincerely, screw yourself.
Maybe he does, and maybe he doesn't. You don't know that because you've only known him for 6 months while he is in an absurd state. You just ASSUME he his (and I personally like to think ALL humans are). Regardless, the ability to steer him from his destructive course or not is not going to be determined by you - it is determined by HIM made easier if he has points to aim for and examples to go by. You offer neither (right now), and you are not equipped to handle ANY of his absurd history or his present spiraling state. You WANT to, and that's sweet... but pointless (as most things "sweet" usually are in this particular setting). You can show sympathy at best... which is the worst possible thing to give someone in his situation. He needs empathy and understanding... people like him working with problems like him and success stories that give him inspiration.
Assuming he WANTS to change (this... is the LARGEST factor in all this... something you have yet to address). With any drug spiral (especially alcoholism) one has to WANT to change... and a world has to be presented BETTER than the drug.
If you come on here and tell his story in chopped up, shock progressive, increasingly dramatic revelations like layers of an onion then you are not here to help him... you're here to help yourself, for whatever reason. You tell us the purpose is to "understand him." Well you can't. So stop trying because it's fairly obvious you are not equipped to digest this kind of situation, yet alone relay the story in one coherent sitting. And even if you did understand it, there is a high likelihood you could get it across to him in the manner and method for him to absorb properly in order for it to take root (something NONE of us can convey over internet anyway).
His being able to digest this information (and there is good information here BTW) is the second largest factor iin all this)
Addiction (to drugs, people, sex, ego, whatever) is like death or love... each version is different for each person, and going through the steps of coping with has to be supported differently. They are all idividualized emotions. If you think of "addiction" as something universal or blanket, then you don't get addiction at all. And you certainly do not get the long history of what got him there... or why this particular path is comforting (numbing in some areas... retaliatory in others). In his abuse he might have found a 'happy place' of comfort while his brother did his thing... he might be trying to recreate that comfort. It might simultaneously disgust him, hence the meth. Or the drugs might be a way of numbing his emotions to retaliate against older men, knowing they will act like you... in which he can emotionally screw them over while sober. It could be he just wants to forget his entire life and wants a force more powerful than him to "jog" that energy out of him...
YOU DON'T KNOW ANY OF THIS and you're not going to understand it anymore than you could understand a 'spirit walk' of the Native Americans or the feeling of an astronaut's first trip to space. So instead of the high possibility of just being sympathetic (and thus condescending) and trying to save him (making him dependent... like the JWs makes their people), without knowing what minor detail in this dense story is triggering this (if any), or (possibly) trying to help someone who has to go through his own personal trial by fire (character development), and without understanding of the many different directions that all look the same from the outside... point him to someone (or an institution) who can help - someone trained in this.
I've seen (literally) hundreds of guys just like you grasping at straws to 'get' levels of damaged human psychology they are not equipped to handle. Assess if he wants to be helped first. If he doesn't, say what I wrote above and do both of you a favor and walk away. If he does want help, guide him to someone who can help him on his level. If he has periods of sobriety, then drugs are not his main problem... think stablization and security first (self worth, maybe). Drugs are the method, not the motivation. If he is drunk all the time, alcohol is a worse issue than drugs - he's in avoidance of something. If drugs and sex are combined, then there is a sexual security he's trying to work with... etc. etc. etc. The many facets of the over-simplified presentation of a very complex story makes it impossible to give any decent advice. He needs a professional, but try to assess which professional he needs first. It doesn't sound like rehab should come first. Therapy then rehab, but that's just an assumption.
You don't have to believe me... but I know this world well. I may be dickish about it, but it doesn't make me wrong. I have seen way too many egomaniacs trying to handle something whose roots go so far and deep into areas they cannot comprehend only to perpetuate the problem. So for "nice" people like you that make matters worse for my younger brothers... the next generation to follow after me... YEAH, I'm going to be a f**king dick about it. GOD is in the details, and it's the details that are important here, and only he knows the details.
This is why I said, it is not your problem. So don't take it on. Kind sentiment does incredible harm when improperly applied. Do the best for him and and yourself - look up LGBT groups (whose main purpose is to help gay "orphans" in this exact situation) and steer him to someone who is equipped to handle this sort of things.
Then lead by example.