Inderal is another drug that has some benefit with anxiety. It's non-addictive and has been around for years and years.

It's something he might want to consider.

IMO, the bottom line is that your influence here is somewhat limited. If he is unhappy he is going to have to take the steps to help himself.

Of course, you can encourage therapy or at least getting him in front of a psychiatrist, but this is his show.

Support, positive reinforcement when he does leave the house, and understanding is about all you can offer him.