They aren't saying that they will go to the parks every day though - OP has said they will be doing the parks/water parks 60% of the time, so just over every other day, leaving plenty of time for Florida's other attractions or indeed enjoying some downtime by the hotel pool.
Also it depends very much what is meant by "doing the parks". In our case we got there for opening time and using the lack of queues and the ability to get Fast passes, also no desire to see any of the parades so we were pretty much done by lunch time, leaving plenty of time to relax.
The exception was strangely enough both Universal where the parks just didn't seem as full on as Disney, so we were there until 5pm at each park.
We also did the Boggy creek airboat ride, Kennedy space centre and innumerable mini golf courses.
If he's going, then he should go along to the majority of the theme park days, but have the ability to bail out should any DC get tired. Other days can be used for other attractions or chilling, but I can only imagine DS's reaction if told that his cousins were all off to Disney, but instead we were going heron watching on the beach
.
I get that some people hate Disney and theme parks. I do understand that these people wouldn't willingly go on holiday there - fine- but if you're getting an expenses paid trip there, or indeed swinging by that location and you have DCs under 10 then believe me the majority of them would like to go and it's just as possible to create happy memories there as anywhere else.
Slightly off thread, but when people mention Disney the favourite Mumsnet response is to come up with some unknown theme park in Holland I think which is meant to have "true magic", because apparently it's not theme parks to blame, just nasty ole commercial Disney.