Hi Guiran,
You ask a very good question. I would recommend two days in the main parks and then hit a water park for the third day. Then hit the the parks the last two days.
I would go to theĀ
Disney website and ask your pirate what he has to see. This way you can plan your days accordingly. It also makes the trip his as much as yours. If you do not get a chance to look on line, purchase a book such as Disney World with Kids to get some thoughts to what he would like to do.
I would start at EPCOT first, then Disney's Hollywood Studios, next is a water park with an evening in either Magic Kingdom or EPCOT. Follow the water park day with a day at Magic Kingdom and then Animal Kingdom.
However, if one of these parks does not appeal to you or your son, mix it up. This is how your research will give you a more personal, enjoyable and magical vacation.
Finally, remember the greatest attractions will be the ones you enjoy together!
Beth