If we had the answers we would be millionaires as a wise man once said. As I see it, at the age of 17 you are no longer a child, although
we know that they have so much to learn. The thing is that it is
his life, you can advise him and council him and hopefully help him if he will let you but you can't live his life for him.
You have known this young man since he was at least two years old. You had such high aspirations for him, you were not going to let him make the same mistakes you saw others make when you were that age, I'm sure you have devoted most of you best years to him, and what do you get in return? A massive smack in the face and a 'Fook you Pal'.
I think you are just about right in your summing up of what you can do. Tell him how you feel, and then tell him how much you love him and that you will be there for him when he needs your support. It may be worth talking to his tutor at college who will be able to tell you if he is having any problems there (You never know).
After that, try not to argue with him, you know teenagers are always right, so there is no point. Respond positively to good behaviour and try to ignore the bad. Bloody hard I know but there you have it.
One last thing, I had a quick google and pulled this out,
http://www.surreycounsellingnetwork.co.uk/scn/contact.htm I hope it helps.
All the best