We did it when our youngest was 9, and again when he was 13. In these groups, there were some kids that had no business being there, panicked in the water, didn't have basic snorkeling knowledge...
If your child is a good swimmer, is comfortable with a mask and snorkel, than I think this is the best experience they...and any non scuba certified adult can have. You get a full half hour in the tank, fish are very comfortable, and even though you stay on the surface, there are plenty of close encounters. You actually don't use a snorkel, but a tank and mouthpiece. You do wear a flotation vest that prevents you from breaking the surface however. If you want any additional details, feel free to PM me.
As Blonde Princess stated, if you say your son is 7 years 11months, they will not allow him to take part. Your choice on how you choose to round...and coach him in his response.