Kudos to you. Don't feel you are obliged to engage any any further discussion with him. A benefit of poly is that it means you should get to surround yourself with MORE love, not necessarily more misery. If somebody wants to be miserable, no reason to let yourself get roped into it one bit once you've freed yourself. Focus instead of the good and positive things in your life!
Happiness will never come to those who fail to appreciate what they already have.