Hybrid,
I am 28 years old and have a brother who also is 17. Unfortunately, I lacked the wisdom to "bail" before baptism. I have been d'fd for about 7 years now. I see my little brother occasionaly and he still speaks to me. We have online conversations a lot and play multi games and such. We still maintain a "hushed" bond, even though he was baptized last year. There have been many times when I have learned of things that he has done, that would get him in serious trouble with the congregation. It's easy to think that if I told someone, I might have my little brother back. But what I have to remember is that disfellowshipping causes so much psychological trauma. Even though it would be a good thing in my eyes to see him out of the cult, there is a big possibiltiy that he may resent me forever, and blame me for the trauma that he might endure. At least now he still talks to me behind closed doors. I believe that one day he will wake up on his own and get out. The last thing I want is for him to blame me for anything. It doesn't sound like your brother is doing anything that could get him d'fd. Maybe you could secretly talk to his girlfriend. She would most likely have an influence on him, if I remember anything about being a 17 year old boy. I hope that I have helped in some way.
Chris