It could be any number of antidepressents, there's a list at the bottom of here...
http://www.rcpsych.ac.uk/mentalhealthinformati on/mentalhealthproblems/depression/antidepress ants.aspx
Just be there for him and listen, try not to pressure him esp if he's being pressured from elsewhere. It can help with someone that you can just relax and have a bit of a laugh with, some respite from it. Reassure him he is not alone, maybe find some local support groups or national ones for people who have lost colleagues in such cirstances.
You could get him a nice card and maybe write how much of a friend he is and such and how you're always there for him and give him the info that way so he can go through it quietly in his own time rather than feeling confronted.
Reassure him it's natural to grieve and be affected by something like this and that it make take time for him to get himself back on track but that you're there for him all the way and will give him all the support he needs, get to to let you know what would help, someone to talk to, get away from it with, someone to rant and get his frustration out at etc... Say things will only be mentioned if he wants them to be so he won't feel pressured or under scrutiny.
If he'd be willing maybe suggest going away for a day or more, preferably where no alcohol is involved, and just getting him away from things, maybe even something special to remind him of the friends he has lost, a bit of closure and peace for him.
Maybe even see if he'd be up for getting involved in something to get him out of the house and give him an incentive to keep going rather than just hiding away.