Sounds a nice idea. If you can’t sail already you probably can’t learn with him in the boat, I’d phone and check that. The lessons are usually in small dinghies like picos. Maybe worth doing a crash course before hand. They may do dart joy rides with instructors that you could do together, not sure if you might be able to book a private lesson or two out there to get you started, the darts (catamarans) are more stable, less easy to capsize than the small dinghies, as long as it’s not too windy. It depends where you go and how windy what is manageable too.
I’d also check about leaving him unsupervised- I’m not sure at what age they’d allow him to be on shore alone while you’re on the water, or even in the pool, they might be quite strict on this. If you could find another single parent in the same position to team up with I think you’d be able to get a lot more out of it.
Kayaks and paddle boards should be fine, tennis courts will need booking and are likely in use for group lessons a lot, again check what age the lessons go down to. I’m not sure how climbing works, probably similar to tennis.
The call centre staff are mostly ex seasonnaires and should know their stuff but might need prompting to think about implications for a single parent family as we don’t see so many, not sure if that’s due to cost or practical limitations with making the most of the activities, but they will try to be helpful. I hope you can make it work for you. If you do go, make sure you go to the big briefing at the start of the week and sign up for your most important things first in case things get booked up, summer holidays are a busy time.