He's been away from you for an entire day on many occasions, right? He goes to school, he's gone with relatives. So he separates well from you, he navigates his space and his social relationships well. That's great - it's partly due to his personality and partly due, no doubt, to you fostering confidence and independence. Now you need to trust that. What is the absolute worst that could happen? He'd miss you? He'd dissolve in tears and be inconsolable? Then they would leave and bring him home, or they would call you and you would go get him. But I doubt that's going to happen.
The only other issue is safety. If they are going to some big water park, you need to address swimming safety and vigilance. Does he have any major life-threatening allergies? Then you need to discuss epi-pens and so on with the other parents. Otherwise, he just needs to know his name and phone number. If you feel insecure, get one of those ID bracelets with your name/phone number on it. They make them for runners who don't carry IDs, for example, and I'm sure you can buy one at children's stores or even pharmacies (they have some with different "charms" on them for different medical issues and you just snap on the ones you need, so you could just use the ID part of it). Identify your fears, then have a phone call or a cup of coffee with the parents, and lay it out there. The more they understand, the more vigilant and reassuring they can be.
I think you should start to rethink the "ask him every time" philosophy. It's great to get the kid's input a lot of the time, but it kind of prevents you from pushing him to try new things. What has to occur for you to be comfortable? Will it miraculously happen at 8? 10? Let him know you've talked to the parents and you feel great about them and the good time he will have now that he's a big boy and can enjoy this big boy treats.
I remember visiting my mother with my young son (about 7) - my aunt died while I was there, and I had to send my son home on the plane to my husband while remaining with my mother for the funeral. I was fine until he marched down the jetway with the airline attendant, and then I lost it. Partly worried (not about the flight, but about him connecting with my husband at the other end) and partly emotional that my little guy was old enough to handle this. My mother reminded me that he was doing fine because I'm a good mom.
You gotta give your kid wings - it's the greatest gift you can give as a parent. If he can't function without you, you've handicapped him. Start small with fun things like a day at a theme park. Then do a sleepover. Work you way up to sleep away camp! It will get you ready for college!