Hi Carolan! Thanks for visiting us here at the DIsney Parks Moms Panel.
The answer to your question really depends on a few factors. Without knowing the age of your son, I can't say for sure what the best plan would be for your family. Also, depending on his age and interests, you may not want to visit every attraction. That will save some time. With younger children, I usually recommend a midday break to allow for some recharging of the batteries, so to speak. If your son is a bit older, you may not feel the need to do this. My son is nearly 12 yrs. old, and he'd rather stay in the park at this age. Having a relaxing table service lunch is enough rest for him.
Whatever the case, four full park touring days should be enough time for three parks. Get to the parks just before posted opening time. You'll be amazed at how much you can get accomplished while others are still in bed. Disney's Animal Kingdom Park can easily be toured in a day, and you may even jump to a different park that night. Of course, you will need the
Park Hopper Option to be able to do this. You can use your fourth day to get back to the park(s) you didn't get to finish touring. Since you stated having the flexibility to add a day if needed, you'll also be able to gauge better once you are there.
Feel free to come back here with more questions. We are so happy to help.
Until then, I wish you a MAGICAL vacation!
-Jacqui