momsupport I'm not sure what advice I could offer, but it sounds like you are already doing a lot. It's obviously how much you care and I'm sure your son knows that... if he feels able to ring you and cry on the phone to you then I imagine he trusts you and feels like you're listening to him. It takes a lot for people to admit something is wrong and I think it's great there's this openness between you. It shows he feels comfortable talking to you and knows you care. So that's good.
I'm not sure how much more you could do. Is there any chance of you visiting him? When I was struggling at uni it always boosted my mood when my mum and step dad came to visit. They occassionally took me for a weekend holiday, that always helped me also because it's totally de-stressing, especially being out in the countryside.

It's good that your son is open to medication and that he's seen a pyschiatrist nurse... is it ongoing though? I think the thing that I would suggest is that he has a regular therapist.