As for what age, you’ll get no end of answers…and all of them are correct. It’s purely what you feel is best. WDW is age appropriate for kids ages 0-100. We brought our kids when they were 4, 1, 1 (the twins hadn’t even quite turned one yet). It was great, we had a wonderful time. Our oldest son understood some of it and really enjoyed it. The twins obviously didn’t understand anything, but it was really cool to see them discovering and enjoying in their own way. All the lights, colors, sounds, etc…were fascinating to them.
Reality is that memories fade as you age. If you take him when he’s 4, it’s not likely he’ll remember much of it when he’s 20 years old. I have almost no memory of being 4. Others may, but I bet it’s not a photographic memory. But that doesn’t mean you shouldn’t take him. I’m very glad we took our kids when we did. Yes, diapers, etc…are a pain, but it was worth it to us. Take him whenever you feel it’s the right time for him to start experiencing it.