I hope, for his sake, that he is able to open up to somebody sooner rather than later. I used to be like him as a teenager, mainly out of fear for me as I was bullied in year 6 and never recovered. One night it all came out in the worst possible way because I had let my problems build up for 9 years, unable to talk to anyone. Also, alcohol is never the answer. If you drink to make your problems go away then they will only come back worse (hypothetical you, of course). I know as I have been there, an event that contributed to my idea of giving up alcohol for the time being.
As for advice, I will try my best but you might want to ignore me. One of my friends knew that there was something wrong when I was always the quiet one and he also knew that I was miserable on my course. On the day I officially dropped out (I'd been planning to drop out for a while before so stopped attending lectures etc) he met up with me and encouraged me to open up about my problems, just by being a friend. I did this and as a result felt much better. I was no longer alone. I think that you should just be a good friend so that he chooses to open up in his time. It took me at least a couple of hours to know that I could completely trust my friend and therefore was able to explain the full extent of the situation.
Sorry if that was a load of rubbish, it's just that your friend's situation sounds very similar to what mine was as a teenager and I do not want him to go through what I've been through.