For clarity - I wanted an adult to be there for time keeping at the pool. To tell him when it is time to get out, and eat. He would stay in long after he is water logged. And his sister is not best at keeping track of time. I wanted them to be able to do something fun. Sure, he could stay in the room playing video games - but he could do that at home. It's a shame there doesn't seem to be any teen age activities at Disney, such as the teen clubs they have on the cruise lines.
If your son needs to be told when to get out of the pool and when to eat, I would def go the babysitter route with the kids clubs.
If you think he just might be able to manage himself, even at the pool, I would set some ground rules for both kids:
1. Go together, stay together, leave together. No exceptions.
2. When you are hungry, eat.
3. When you are cold/tired/waterlogged, grab a towel and leave the pool.
4. Hide your phone in your bag at the pool while you are swimming, otherwise, I expect you to answer at any given moment.
Again, unless there are issues notwithstanding, kids are pretty good about knowing when they have had enough, and when they are hungry, by age 12. If you do not do this already, test it...give your son the freedom to make some of his own choices for a weekend...when to eat, what to do, when to go to bed. He might surprise you. My 9 year old will stay up all hours and play video games, and will not eat unless he is hungry. When he is hungry, he eats. I don't worry about it a whole lot. (and for you...it's maybe ONE missed meal if noone is around your son and he decides to skip eating for extra pool time). My 14 year old can pretty much live on his own at this point...he eats, sleeps and exercises when he wants. We are pretty much at the point where I have given up forcing him to do anything not crucial, but he is a good kid with good grades and multiple sport commitments). My 11 year old pretty is much following suit to her her older brother.
If it is really bothering you that he needs something to do while you and your DH go out to eat, and kids clubs/babysitters are not an option, maybe put V&A on hold for a few years and have a family meal instead?
And, there are a gazillion "teen" things to do at Disney...just BEING there is a thing. Enjoy the parks and resorts!