Hi there,
Well, seems like your level of trust actually isn't too far off base.
Maybe he is just young and easily influenced by his buddies and still kind of excited by the many options out there.
No need to wait for proof that he is seeing someone else, or that he's a bad guy, or that all men are unfaithful. You can decide for yourself what you want and what will make you happy.
Intimate relationships, if you would like one that is monogamous, take a lot of attention and care and tending and focus. It's hard to have one while also spending that much time window shopping.
What are the life dreams that you want for yourself?
Once you clarify that, you can more easily have interactions (all across the board) which honor those dreams and support your potential.
Best wishes,
A