I wouldn't really know, just speaking what I think, hopefully, it's not depression.
Well, gf of 4 years, and doesn't speak to her? I am more inclined now to believe he has something wrong.
Like I said, make an intervention, gather the family, friends, gf, and who ever is in his life, and make a conversation with him, with all those people present, figure some topic to discuss and then get to the point where, you all, ask him what's wrong with him.
With many people, he will see how much everyone loves him, care for him and wants to see him well.