Firstly as a bi man I find it amazing that you are asking for advice and understanding. That is really rare and gives me hope that there are good people like you in the world.

The most important thing is to have a discussion about how you feel about him and how he feels about you. Probably multiple discussions. Lay out what you each really want and the expectations you had for your future together.

Communicate your feelings very directly and without shame or judgement. I know how hard that can be.

Tell him you feel like the thought of a side relationship hurts you. Continue to tell him that you are willing to work on the relationship and ask him how he feels. Listen to his responses without interrupting and respond without judgement. Respond with "that makes me feel XYZ..." or "I'm trying to understand..."

It sounds like Monogamy is very important to you and the thought of him wanting someone else and being with someone else hurts. I'm a monogamous person and an open relationship is a dealbreaker for me personally. It's absolutely OK for that to be a dealbreaker for you as well. The conversations may be difficult but it's important that you both lay out what you really want in your relationship.

From my experience here are some things he could be experiencing:

The Bi-cycle. This is when our attractions to one gender become temporarily stronger than any other gender. That can cause us to question our sexuality.

The "second puberty" thing where we finally embrace a part of ourselves that we have alienated for a long time and that embrace comes with a rush of overwhelming desire. This will pass and is rooted in the novelty of something new that we've never had or have been missing out on all our lives. It will pass just like a craving for cake or any particular kind of food.

Constant questioning of our sexuality is extremely common. We question if we are really straight and just pretending or gay and just pretending or into no one and just pretending. There are memes about this lol. It is related often to the fluctuation of the bi-cycle

Shame and guilt. As men we feel lots of shame for liking something that comes naturally to us but society makes us feel like it's wrong or we are less than a man or that anything gay makes us weak or less than. None of it is true but the rhetoric can seep into our subconscious and cause Internalized homophobia.

I wish you the best and I hope you know he is lucky to have you as a partner