How do you just trust that he does think I am his priority and wants to stay with me?

Start by accepting the possibility that he may not always think you are his priority and want you to be his most important partner. Nonmono relationships evolve over time, and if he meets someone who is more compatible, he may decide to commit to that person in ways he no longer wants to commit to you.

And if you meet someone who is more compatible, you may decide to make that person your primary relationship, while M25 becomes another kind of relationship.

If you can admit that might happen, to either or both of you, then you can start thinking about what that would mean for you and your relationships - whether that is a risk you can live with, what you will do if it happens, and what you should do now based on the possibilities.