I have a 19 year old cousin who was talking to me about something similar the other day going on her life. I will give you the same advice I gave her, as it seemed to help.
First, you are still a very young woman. This isn't berating you or saying you don't have depth of emotions, as you obviously do seeing how much you care for this guy. However, you are young to limit your options so much by committing to one monogamous partner when you feel that you are truly a polyamorous person. But you need to be honest. Just as you wouldn't want or may come to resent your love life being limited by having one mono partner, you wouldn't want this partner whom you care for to resent you for dishonesty or the cheating that could occur down the road. Be clear in what it means to you though, as many people take it to mean "I want to have multiple sex partners" and nothing more. You may find that you don't meet anyone else you're interested in for months or years. You may even find that even though poly is in your nature, you are happily living monogamously with this man for a long time to come should the right other not come along. But he should know the possibilities.
I think you've gotten good advice from the other posters. Congratulations for not waiting until you're married, have a child, and end up cheating before coming to accept yourself and wishing to share with your love.
Good luck. I hope it turns out well for everyone involved.