If you feel you want him all to yourself while he is poly and partnered, you are being very unrealistic and unfair to both him and yourself. Don't be a cowgirl.
Besides, you've only been dating him for four months. That is no time at all, and way too soon to move in. Obviously, neither of you is thinking straight, probably due to the rush of chemistry you are experiencing (endorphins, dopamine, oxytocin, etc.). Slow the fuck down, wake up, get your feet back on the ground, and get to know him better. Sheesh.
Meantime, visit this site: http://www.reuniting.info/science/sex_in_the_brain