Hi Katrina!
Chef Mickey's is a personal favorite of mine, but since you already will get a chance to see the "Fantastic Five" (Mickey, Minnie, Donald, Goofy, Pluto) at Tusker House, I'd say take the morning Extra Magic Hours at Epcot.
It'll be a great opportunity for you and son to get on rides before the crowd rush (i.e. much smaller wait lines) as well as "tag" some of your favorite rides at the beginning.
Some of the favorite rides he may be able to catch in that morning Extra Hour include:
- Journey into Imagination with Figment
- Soarin
- Spaceship Earth
- Test Track
- Mission: Space
These were favorites of my daughter at 5 (especially Soarin'), so you can look to catch those right away!
Have a splendid time at Disney!