Ask him and trust what he says. Its all you can really do. I worry about this sometimes too but I have to trust in my partners that anything I am doing is not hindering them as people.
I would also second this
No relationship is guaranteed anything,...what matters more is if you feel you can handle yourself. Are you prepared to enjoy the relationship for what it is, and what it has to offer, and letting the cards fall where they may ?
I think one of the biggest challenges people have in relationships is traditional expectations. I find relationships work better without them. An expectation is like an assumption, sometimes you need to clear the air and work with goals together. If they don't match its time to start communicating.
One of my biggest weaknesses is my own brain, I develop expectations and think 4 or 5 steps ahead. This always leads to disappointment. I am working my butt off to work in the now, lay my dreams on the table, and hope the people involved want to work with me to achieve them, if they don't then other dreams have to be made.
Best of luck and enjoy the journey. If you look for mono's earlier posts on this forum he has documented the struggles of a mono coming into a poly relationship, well worth the read