I have been/am in a similar position with my daughter, although she's still living at home so in a sense it's been easier to deal with. Our experience was that 16-18 year olds fall into a medical gap. They're considered old enough to see a doctor independently and yet aren't old enough to fall under the adult mental health services, whether private or NHS.
I think it's important that you don't just leave him to it because he clearly needs help, but you will need to tread carefully. Perhaps you could get some advice from one of the specialist organisations: