It's really hard for you. You're seeing your son suffering and can't sort it out. I think it must be immensely difficult for parents in this situation.
I think your son has definite drinking problems. Excess drinking, even though it isn't constant, is a serious problem. It could also be affecting the uptake of the sertraline.
In England and Wales the charity MIND have centres and a phone line. It might be a good idea to give them a call. I would suggest to that you get in touch with the mental health services in your area to see if there is a carers group or any other help. I have found that you have to be like a detective to discover the little help there is. But the more different groups, people, helplines you get hold of the more likely you are to find a way forward.
I'm sorry I can't help you more