Hi Krista,
Honestly, for a 5 year old, I would go with Animal Kingdom Park hands down. The
Kilimanjaro, Maharajah Jungle Trek and Pangani Forest Exploration Trail are all "must-see's" for my kids. Your little boy would likely enjoy the Boneyard as well. Our family loves the Animal Kingdom Park so much that we always try to schedule 2 days there. We find that taking time to enjoy the animals is a completely different experience.
I know that a lot of people are big fans of Hollywood Studios for little ones. I'm not. Don't get me wrong,
I love Hollywood Studios and it's my older kids favorite Park. However, when my youngest was 5, she opened my eyes to the fact that there isn't much there for little ones. They are too small for most of the rides (except Toy Story Mania), and a lot of 5 year olds don't want to sit still and watch shows all day. Granted, there are some interactive shows, like
PlayHouse Disney- Live on Stage. However, when I asked my little girl if she had fun at the end of the day (when she was 5), she said, "No, I don't like food". I realized then that all she had been able to do that day was eat. She was too small for the majority of the attractions. She munched on popcorn while the big kids had a blast on Tower of Terror, Rock 'n Rollercoaster and more. She did watch Beauty and the Beast as well as The Voyage of the Little Mermaid. However, she didn't remember the shows though...she just remembered that she was too small to do much. Contrarily, she loves Animal Kingdom Park and is completely fascinated by the hippos, gorilla, tigers and more. As she gets taller, I'm sure she'll come to enjoy Hollywood Studios much more.
Just my 2 cents. Hope this helps and that you have a wonderful vacation,
Kaylene