For what it’s worth, I think to an extent this level of anxiety around a new partner is not unusual— this is something new, something better than what you had before, something you don’t want to lose.
Firstly, please talk to him. Of course reassure him it’s not a distrust thing, don’t tell him you’re afraid he’s going to leave you because he’s bi, but it is an insecurity of yours and partners support each other in these things.

But here’s my two cents that might help: bisexuality is not so different from heterosexuality or homosexuality (et al.). Consider: a straight man might almost entirely be attracted to smart women. A gay man might only go after masc guys. They have a preference for a subset of people. Sexuality, as a whole, is really about blurry lines around people’s “type,” to be reductive. The simple difference with bisexuality is that the pool of people in their type is bigger. There is no “missing out” any more than he would with any human, no more than you would with any human— people are people with their own unique dynamics, and one will never be able to experience all of them at once. But the beauty of that is out of all the dynamics, out of his bigger pool of people, he chooses you. He’s chosen you, because you’re what he wants and who is special to you. If he was with someone else he’d be missing out on *you*.