I was very close to a family for 15 years.
Mum and Dad split up which caused Dad to spiral into mental health crisis after crisis eventally ending up in his death due to alcoholism after a terrifyingly short but drastic decline which was obviously horrendous. Kids were 14 and 11 at the time and there's no other family apart from 2 uncles (on Mum's side) who have been less than useless the whole time.
During his decline and death, I accepted my own creeping alcoholism and am now 7 years sober. Mum continues to drink, probably has her own mental health issues and I've had to detach, with love, for the sake of my own sobriety.
The eldest DC is incredible, got herself to Uni and is doing really well. I have stayed in touch on messenger and told her that I am in recovery and will always help her mum if she ever asked.
Youngest DC turns 18 in a couple of weeks and is the opposite. 
He's had limited parental input since Dad died basically and although still attending college is telling his sister that he's stopped eating (too scared?), isn't looking forward to life, doesn't want to get any older and that he hates his college course and doesn't want to do it anymore. He's got no money on his college card so isn't eating there and eldest DC says Mum won't put an effort in to get food in to eat so there's limited options at home.
After the DD's last message to me where she directly asked for my help, I now have to try to do something and I am clueless as to what my next steps are and what I can expect?
I'm thinking his college could be my first stop - so trying to find their safeguarding lead and telling them everything I know and hoping that they are aware and supporting?
Me and his sister will also try to push to get him a GPs appointment.
I will try to connect directly with him so have asked his sister if he would speak to me or meet for a walk.
DH and I would have him to live here in a heartbeat if he wanted to and Mum would consider it. We are closer to his college and have the room. Although I admit I am probably very naieve about what that could entail.
Can anyone give me any advice on how I can best try to support him whillst he is in this toxic environment and I'm not sure he will even talk to me?