Savygirl, I tend to agree with the other answers that have already been posted - the idea that someone try to place rules around another person's feelings is quite a bad warning sign.
This prompts me to ask - have you met his girlfriend? How do you know that they have an open relationship? Is it just because he has told you this? Have you actually been able to verify this?
If this is just about sex, then this may be an "open relationship" but it's not polyamory, which is everything about love (and the sex that goes with the love).
My guess is that you have already kinda of fallen in love with him, yes?
"Listen, or your tongue will make you deaf." - Native American Proverb