I'm not going to tell you not to worry, because if your son is on anti depressants, is drinking and has suffered from depression in the past, I do think you are right to be concerned.
I suspect you are worried that he may accidentally or deliberately end up taking his life by overdosing on a combination of medication and alcohol, and if you are genuinely concerned about this, I think you should open up to him and share your concerns. If he has suffered suicidal thoughts in the past he may not want to admit it, but if you seize the initiative, break and taboo and say "I'm worried about you feeling so low and that you might be thinking about suicide because you feel there's nobody you can talk to ", he may just possibly feel that he's been given permission to talk about that option. If that is the case and he doesn't want to talk to you, at least try and persuade him to talk to his GP or The Samaritans. You may find it helpful to read "Choosing to Live" ISBN 1-57224-056-3 which is full of helpful advice both to those who have suicidal thought and those who are trying to help them.