Hi Brian!
I'll answer the easy question first! Based on my experience from taking my 5 and 10 year old sons every year to the parks since they were babies, I think your son will enjoy all of them. However, I think he will definitely enjoy some more than others. My recommendation would be to concentrate on the Magic Kingdom, Disney's Hollywood Studios, and Disney's Animal Kingdom and in that order. If you have any time left over then I would visit Epcot.
The Magic Kingdom is ideal for young guests. With the exception of the Haunted Mansion and Stitch's Great Escape, my 5 year old loves everything there that he's tall enough to experience.
At the Studios, my 5 year old loves Muppet Vision 3-D, Playhouse Disney - Live on Stage, Toy Story Mania, Voyage of The Little Mermaid, and the Beauty and the Beast stage show. He also enjoys watching the Jedi Training Academy. He also enjoys Fantasmic! which is the nighttime spectacular at the Studios and something that should not be missed!
At the Animal Kingdom, my 5 year old loves Kilimanjaro Safaris, the Boneyard play area, Tricera Top Spin, Festival of the Lion King, and Finding Nemo - The Musical. Be careful of It's Tough to be a Bug! That attraction scares some very young guests.
At Epcot, my 5 year old loves Maelstrom, Soarin', and the Seas with Nemo and Friends. He also enjoyed Spaceship Earth and the Gran Fiesta Tour Starring the Three Caballeros.
With regards to tickets, you can get the latest prices
here. On the right side of that page you can enter your zip code and see the special ticket and pass prices available only for Florida Residents. Since you have never been to Walt Disney World before, my suggestion is to start off with a Magic Your Way ticket for the days you will be there and then you can always upgrade to another type of pass, such as an Annual Pass, before you leave.