New guy keeps bringing up ex

I’m dating this incredible new guy but the downside is that he keeps bringing up his ex. We’ve gone on 3 dates but talk on the phone a lot and he brings her up (always in a negative way, not disrespectful just to say why it didn’t work out). We both broke up with our ex’s earlier this year. For me it was June and for him it was March I believe. He even mentioned that he is an open book which is why he isn’t afraid to be open and honest but it does kind of bother me since I don’t bring up my ex unless he asks a specific question. How can I nicely tell him to stop?

EDIT:

On our last date he almost called me his ex’s name when he was trying to say shut up in a joking way but I ignored it and I understand that his breakup is just as new as mine. We both dated our ex’s for 3+ years and I’m sure it’s an adjustment for him to be with someone new. I just really want him to stop talking about her as I think it will negatively impact our relationship