This is almost a plea more than an opinion.
This is coming from my own thoughts as a monogamous person.
He may be open to the idea and he may decide to end what you have right now. I would suggest having him look at this forum to get an idea of polyamorous relationships and make sure you are clear on how you define your own poly nature.
Don't let him commit more of his heart to this without knowing your heart. It may cost you an intimate relationship but it may promote growth in it. Either way, being honest should leave you with a healthy friendship if this is not the path for him.