You need a verb. It should be 'please BE nice to him'. This means treat nim nicely. 'Please be nice with him' wouldn't cause any confusion but natives don't say that.

Please be nice to him. It's a statement to show respect to others.

1. Please be nice to him. This is the more common and natural way to express the idea. It's a straightforward request to treat the person kindly. It's used when you want someone to show good behavior, politeness, and consideration toward another person. 2. Please be nice with him. While this phrasing is grammatically correct, it's a bit less common. In English, the typical preposition to use after "be nice" is "to," as in "be nice to someone."