I think that you need to get yourself 100% sure of why you had a problem the last time, and know in hour heart of hearts that it won't happen again, or at least be confident enough about it that there is only a very small chance of it causing a melt-down.
Because the "other person" is ALWAYS going to have something that you don't. More beautiful, bigger breasts, better figure, more intelligent, more socially adept, higher sex drive, nicer house, better friends... and the list could go on and on. He is bound to find some element of a relationship that is different from the one you have with him - otherwise why would he be with anyone else?
If you want to truly be poly then you are going to need to understand how any of these will affect you.
...and all of this is BEFORE you talk with him about it again.
Then, when you talk to him, you can explain what your issues were that last time, and why you feel there there is a significantly lower likelihood of them happening again.
Try to do this independent of any other specific love interest - this should be a fundamental part of your relationship with him. When there is another specific person "in the mix" it just muddies the waters even more.
What he is doing is probably the same thing I would do in this situation - he loves you, saw just how much him being with someone else hurt you, and doesn't want to do that to you again.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb