Hi Meagan!
Thank you for bringing your question to the Disney Parks Moms Panel!
I am actually at Walt Disney World right now and had that exact same question from a friend planning a trip next month. So, having been to both parks this week, I will tell you what I recommended to her.
I have four teenage boys with me and we were at Hollywood Studios for about four hours and they felt like they were finished. (keep in mind that they didn't want to see Fantasmic!, so that wasn't an issue)
I really feel like your day would be better spent at the Magic Kingdom. Since the crowds may be higher than normal due to your trip being during spring break, you could easily spend the entire day there and be able to do what he wants without feeling rushed. In my opinion, the time traveling between the parks would eat up too much of the day. If it were me, I would get to the Magic Kingdom early and plan to be there until it closes. We have been there twice this week at closing time and it's amazing how much the crowds thin out and how short the wait times are for attractions.
Your son can start planning his day by looking at all of the
attractions available at the Magic Kingdom.
Either park will be a lot of fun, but that is my personal opinion!
I hope this helps! Please let me know if you have any other questions.