Set boundaries.

Having a conversation with him at an appropriate time, probably not when he's bringing him up, could help.

Perhaps take him out for a cup of coffee and have a discussion with him about how you need to move on and continually referencing him and berating him isn't doing you any good.

Why does he do this?

Well I don't know. But he seems to need to keep reminding you of what a jerk your ex is, for whatever that reason may be. Perhaps he never wants you running back to him ... or (less likely) feels like he is "supporting" you.