hmm, LDR's are a tough one. I suppose you ought to really try to look at his side of things, being shy, introverted, and not previously interested in poly/openness, he has found someone he is finally comfortable enough with he can express that with. so much so that he is willing to bend for you as well.
That being said, he also would have to come through with his side of that promise, and you would need to hold him to it. that doesn't mean you cannot have boundaries, it just means you have to break down a barrier and establish new-mutually agreed upon boundaries.
Here's an example: The girlfriend and i have a boundary/agreement (for now, this may change in the future) that a new person which we want to date/become intimate with has to come home (for dinner or whatever) to meet the other half BEFORE anything besides flirting and kissing happen. It works well to help both people respect both sides of the relationship. As well as to let them know, "our relationship will NOT take away from the primary relationship.
Oh, and another thing, BE HONEST!!! Those scared feelings, guilty, insecure, etc. ARE OK. Just because those feelings are negative, does not mean they are bad.
You say you don't want him to "miss out," well, how much are you willing to let him miss out on because of insecurities? And visa versa? Your relationship has to come first, anything outside of that (for now) is extra, but sometimes in life the "extra" is of dire importance as well.
It is right for you to worry about that "blind NRE." Why does he want this all of a sudden? My best advice here is TALK!!! In any type of relationship, transparency is the name of the game. Anything less is not quite love. Let him know how you feel, and that you will expect the same freedoms as him.
Best luck with it all.