Why would a Narcissist never say one bad thing about his ex to his new girlfriend?

He can't. A man can't say anything about a good woman, to do that would be a hypocrite. He knows you were good to him and to say anything bad about you would make him look like the bad one because he let you go or rather you let him go.

AnswerI think the Word "narcissist" has been wrongly used in a lot of cases and especially in this case. A narcissist would not only not admit such a thing to anyone else, but is in denial about this to themselves. I think other things went on (each relationship is complex) and you split-up. He obviously still cares about you and admires you. He sounds like a decent enough guy because he sure has had opportunities to bash you and hasn't. This guy is giving compliments. Doesn't sound like the narcissistic type to me.