I would add to what has already been said that building up your H and showing him he can derive what he needs from himself, with your help, you make outside influences less effective. Showing how grateful you are, how he is not a second choice and apologizes for specific things really go a long way.
You need to talk to him about it, but in a very gentle way. Just because he is a BS does not mean he is going to take this friendship any further. I think you realize it is much more individual than that.
Aid him in repairing his self esteem by trying to woo him a little. Speak his love language, show him without a doubt that you are "all in." Compliment him to other people when he thinks you don't know he is there. You get the idea. He is desperately trying to get someone to disagree with all the messages he tells himself in his head. You are in the best position to help him with that.
It is hard to do, but you will have to address your own insecurities as well. The projection of your feelings and experiences also play a factor here and you need to be cognizant of that. Not every friendship turns into an EA.
When you are at a low point the lure of attention is hard to ignore. He needs to be mindful of that too. IC has helped me a lot. Talking to my W about how much it hurts to see me react to her choices was just as, if not more, effective in realizing that the attention I get was unhealthy and really did little to make me feel better in the long run. Accusations that my W made actually drove me further away and made me feel that she was saying "Do as I say, not as I do," Avoid that at all costs. Approach from how it makes you feel. (use "I," not, "you."
Ask him about his friend, but pair it with wooing him and building him up at the same time. It won't happen overnight, but having anyone paying positive attention to him right now is important to him. Be that person.
DS 1, DD 6
Dday 8/31/11. ONS that occurred 3 years earlier. Lied to for 3 years.
Every truth comes to light in a long enough timeline.