Maybe ask him about his views on relationships and the idea of polyamory. He may not want relationship with you if you are expecting a monogamous relationship. If you cannot deal with the thought of him being emotionally and physically close to another I would not pursue this. It may help to understand what he wants versus what you want and see if there is still compatibility.

My good friend practices polyamory and has several partners. All of their partners know of one another and practice it themselves. It takes an immense amount of trust to partake in these types of relationships. They have also been in some of their relationships for several years so commitment is realistic.