There is not much you can do for him except be there for him. Don't try to overdue the cheering squad (that brings feelings of guilt to him which doesn't help), just be yourself, that's who he fell in love with. Just being there for him with a hug or a smile is what he needs, even if he doesn't acknowledge it.

He's lucky to have someone who cares enough about him to even be on this site to try and understand. He does need professional help, but he will get it when he feels ready and able to admit it even to himself (the hardest part).

When someone goes in their "way down" mode, there is nothing you can do for him, just love him and remember, only time can help him, but your love will get him through.

Best of luck Sweetie.