"Hey man, I noticed that you're kind of a dick to yourself. You're always saying something bad about yourself and calling attention to it. Is everything ok? I hope you don't think I'm thinking those things about you, I enjoy having you around."

Sounds like insecurity to me. He's worried he's a fuck up and everyone else thinks it, too. If he calls the mistakes out first others can't expose him and humiliate him. As a friend it's good to know how your friends need to be reassured/shown love and acceptance. Sounds like some verbal reassurance could help him, point out things he does well, compliment something genuine - not something forced or fake sounding, show him some love. And if the group is talking about it, and the group cares for this friend, the group all bands together to build him up not make him feel worse. And then, when he's ready, you shit talk him as usual and harmony is once again achieved.

You're a good friend. Good luck.