Hi Belle,
K. is right on about the ground rules. They are absolutely crucial. You two are venturing into uncharted waters for your relationship. Bringing in another person or couple into your sex life will really test the strength of your relationship, as well as your own confidence in yourselves and your partner. If you have any jealousy issues, be very cautions, and try to work through those first. You don't want to be having sex with another couple when you figure out that your man "has no right touching that woman!".
So, yes, the ground rules will change after your first experience, and probably after each of the first few experiences. But this is NOT the time to be asking for forgiveness instead of permission. Make the rules, and stick to them. The day after is a perfect time to readdress any of the rules that no longer apply.
Check out these videos we did recently about how to venture into swinging slowly and cautiously so it's actually a fun adventure for you two and nobody gets hurt...
Also check out our Swingers & Threesomes Channel for a lot of other helpful articles, videos, tips, and advice you'll want to be familiar with right about now...
As for his enthusiasm, your guy is understandably excited... after all, from what we've generally seen, many couples end up venturing into swinging with other couples as a result of the quintessential male fantasy of a threesome with his woman and another girl. But their girl is generally less threatened by a couple, where you tend to know who's going home with whom when playtime is over. Who knows how long he's wanted to try something like this, and now that he knows he's about to get it, he wants to see it happen... NOW!
Just be sure and be selective of what people / couples you get together with, don't let him rush a meeting with someone you wouldn't normally want anything to do with. Your first few times will be a great learning experience of what you want and what you don't want, but you should still be smart about it - always be informed.