To the OP...
And to the OP, be careful. I'm Mono. I knew full well that I was going into a Poly relationship, and I went through some VERY hard times regardless.
Being monogamous, I am NOT going to have another partner to fill the time when my partner is away. In my relationship, we don't really have a hierarchy of sorts - his OSO and I are at the same "Primary" level, if we want to put a term to it. If he's not going to have a relationship with you that is anything more than part-time, and he's Mono, he may eventually have a big issue with that as the relationship develops and he gets closer to you, since the Mono tendency is to spend MORE time with a partner as the relationship gets closer. If there's no more time to be had, it can feel like a barrier is in the way of creating something long-term, and if he has nobody else, it can feel very unfair.
His mileage may vary, of course... Coming out of a divorce is never easy. In my case, I discovered some codependency issues that made all the above much worse to slog through and figure out what the heck my problem was. He may need you more as a friend for a while, so he can get back on his feet, emotionally, and find strength in himself before looking at another relationship.
Hope this helps!