When he bashes on your ex try saying something like, "Fortunately, that's all in the past." It's a kind way of telling him it's irrelevant and need not be a frequent topic of discussion.

If you co-parent with your ex, the topic of your ex is regularly going to come up because you'll always have ties with him via your child(ren). If that is the case, be more patient with your boyfriend, but be upfront that it doesn't help you that he disparages your ex.