I would add that perhaps it would be helpful for him if you guys were to start discussing what the future of your relationship would look like. The fact is that if he is indeed mono and unable to be anything else, he would have to have a clear understanding of what he gains and what he would have to give up when pursuing a relationship with you. There's a hard truth that if he's mono, he'd have to give up a lot to stay in that relationship.
However, if he's starting to explore the possibility of not being mono, then it also makes sense to discuss what that relationship could develop into. I honestly don't really know "how mono" this guy is, but I don't think that monogamy has to prevent him from understanding the poly boundaries you want.
Either way, you brought it down to the clear question: Does he want to explore staying with you while being in other relationships or does he want to find a lifelong partnership to replace the one you're building. Both of these are fair ways to move forward, it just seems he needs to figure out for himself a bit more of what he really wants. And it's completely right of you to be clear that you do not want to be a placeholder for relationships in his life. Either he commits to this type of relationship with you or he doesn't. Giving him as clear a picture as possible about what a relationship with you can be and can't be will help him figure that stuff out.