Anorexia, bulimia and body dysmorphic disorder are all serious medical issues and are not limited to just women or just people of a certain age. I certainly can't diagnose what the problem is, but I agree that something isn't adding up. If he's lost that much weight without exercise and while binge-eating junk food, there's something serious going on.
If he is bulimic, he's doing very serious harm to his body and it needs to stop. How it stops is very much up to him. He will either see what is happening to him, or his body is going to fail. Hopefully, he understands that he is suffering from a disease before he succumbs to it and winds up with irreparable harm to himself.
I can't tell you to have an intervention with him, but I would suggest that you talk to his family and friends and see if they have noticed what you are noticing. Hopefully, between you and the rest of his support system, you can come up with a course of action on how to help him.
I really, truly wish you luck. Please keep us all posted.