I'm going to ask my boyfriend to stop talking to his ex.
Why don't you want him to talk to his ex?
Also I'm gonna fight with him for going out with her for dinner.
You can't control who people talk or interact with. If you are uncomfortable with  your partner talking to their ex, you should let them know how it makes you feel. Trying to control them will only make them resentful.
Who said anything about controlling him? I just want him to stop contacting her.
Fighting with him will make it worse, it'll damage your relationship. You better talk to him in a civil manner to let him know that you dislike his meeting with his ex.