My son's (19M) friend (18M) wants to stay with us to finish out the school year instead of being sent to his grandparents but I'm torn on whether to take him in

So my son (HS senior) has a friend who has been staying with his aunt/uncle for the entirety of his HS career (2 1/2 yrs now) but because of friction with his aunt/uncle, they are wanting to send him back to live with his grandparents out of state. My son & his friend asked me whether he can stay with us so he can finish off the school year here because he does not want to go stay with his grandparents. We've only talked to the kid, not the uncle/aunt. And I don't know the kid at all, other than talking to him last night.​According to him, his mom has psychological issues and either sent him to live with his aunt/uncle or they took him in based on the situation with his mom. Not entirely clear about this. His dad is out of the picture. It's unknown to me whether they wanted to do this or felt obligated to do so. In any case, in the 2 years he's lived with them, they've wanted him to do specific things (apply to a boarding school), join the military, take online classes to graduate early, etc) which he's refused. So now they are washing their hands of everything and wanting to take him to live with his grandparents out of state. I feel like he's being the typical obstinate teenage kid and the aunt/uncle don't really want to take care of him and that's why they wanted him to go to a boarding school or try to graduate early. And since he's not doing what they're asking of him, they're passing him off back to his family.​So my son & him have come up with a scheme to have him stay here until he finishes HS (this spring). Their plan is to join the National Guard & go to college. They've come to ask me whether I would be willing to do this. They have hinted that he may be able to stay with another friend & his family too, but that's not a done deal either. In any case, he will be doing this against his uncle/aunt's wishes and they will likely cut him off if he does so.​I am reluctant to do so, and I'm sure it will upset and be disappointing to my son. He doesn't have many friends and he could see this as not being supportive of him & his friend. I have 2 other kids (girl 8, and son 13) and I honestly know zero about this kid. My family's safety comes first and bringing a complete stranger into the house worries me. I am also divorced with joint custody of my kids so he would be splitting time between me and my ex-wife. His aunt/uncle also will be taking his car away if he does this so he'll not have any transportation and will have to rely on us to do so. I'm in a committed relationship with a girlfriend and she really doesn't want another kid to take care of. And same goes for me, I really don't want MORE responsibility for another person, esp. someone that isn't family.​Legally, what would be my responsibility for this kid? I mean, he's 18 so he's legally an adult. But what if something happens and he has to go to the hospital or whatever? Also, it's not that I don't trust this kid, but I know nothing about him and I don't feel like it's entirely 100% safe to bring him into my home where I have younger kids. I might feel more at ease talking to his aunt/uncle but I don't think that will be a possibility.​Of course I want to do the right thing and help this kid out but I need to keep my family's well being the highest priority. What do y'all think? via /r/Parenting https://ift.tt/2qMAhz6