Hi Jennifer,
Congratulations on your little guy's first big visit to Walt Disney World! Personally I recommend spending two days (at least) visiting Magic Kingdom for children under age 10. Even after spending two days in this iconic park you will be surprised at the number of things you haven't done and places you haven't explored yet. To help you plan your son's first visit you also might find the information in the following link helpful:
Planning for Preschoolers & Little Ones. Located within the link is a downloadable "Preschool Parent's Guide to Walt Disney World Resort" with helpful tips and planning advice.
When planning which rides and activities your child would like to do in each theme park I also recommend using this
Walt Disney World Attractions link to help you decide. I have pre-selected Magic Kingdom as the theme park for the displayed attractions list and you can further use the filters located in the middle of the page to select items such as "height" and "thrill level". I strongly suggest measuring your child's height so that you will know which attractions he is eligible to board. From there you can choose which attractions appeal to his personality. My 5 year old boy especially loves Buzz Lightyear's Space Ranger Spin, Peter Pan's Flight, Splash Mountain, Big Thunder Mountain Railroad, Pirates of the Caribbean, Mickey's PhilharMagic, and 7 Dwarfs Mine Train as his top Magic Kingdom attractions.
Thank you for stopping by the Disney Parks Moms Panel Jennifer. Please let us know if you have any further vacation planning questions, we are always happy to help!
Blessings,
Brenda ºOº