Your son will love CFW, my son was 11 when we first went there and we have stay there every time. You need to book the buggy (it costs extra), the buggy is the thing that made it for my son. The pool is great too. You can watch the electric parade at night from the beach and also sort of see the fireworks. I think there are boats you can hire to fish (not sure on this last one might have changed since our last stay). You can take the ferry to other resorts and ride the monorail and visit other resorts (my son enjoys visiting the games rooms at different resorts). There are ponies he can see, there are horse rides as well (not sure if old enough).
My concern that it is a birthday trip, are you sure he is not going to hope he gets to go to the parks. I would definitely do a park ticket rather than a fireworks cruise. The water park is a nice idea though. You could do Hoop Dee Doo, he would love it, it is such good fun and so much food including fried chicken - but again I suspect he would prefer a park visit if a choice had to be made.
AKV is also great but my son preferred the buggy and cabins. Does have two great pools. The animals are fantastic. Does he love animals? There is a mini safari but again it costs money, the question would be if you are paying for all the extras would a park visit be more fun. Only you know your son and what he would enjoy most.
Thats about the price as a single day theme park.
