Hi Brandy!
The Magic Kingdom is #1 for a child who's 5 years old. But if you're looking for the next best one at that age, I'd go with Animal Kingdom.
It's hard to find any kid at 5 years old that doesn't absolutely love animals. In terms of impressions, I think Animal Kingdom had the biggest impression on my daughter right after Magic Kingdom. We've visited when she was 3, 4, and 6, and she still frequently talks about her experiences at Animal Kingdom. Some at the top of the list include:
Kilimanjaro Safari-fantastic voyage into the heart of Africa. For a few minutes, you'll feel like you're really there. Kid friendly attraction.
The Boneyard-couldn't get my daughter away from this. If your child likes playing with sand/digging up things, he will love this!
Discovery Island Trail, TriceraTop Spin, It's Tough to be a Bug, all are cool experiences your child should enjoy. The Jammin' Jungle Parade should get him moving as well!
Hope that helps in your planning! Have a super magical time on your trip!