If you have any kind of theme/amusement park or midway close by, you can get him in training, so to speak. My son was 4, a few months short of five, when he started going upside down on rides, and that was due to getting him familiar with the concept, and, as has been stated before, respecting his limits and understanding that all kids are different.
He went on a log ride the first time at 2, and was willing to go again the next day when given a choice (the first of our key chain photo collection). At four and 44" we walked up to a simple out and back coaster at Paramount's Great America with a single loop, but had to back off because it went by as we approached, and the launch was loud. This is a good example of different things with different kids, sounds affect him a lot more than physical motions i.e. roaring Yeti on the Matterhorn. But later that same day, we went on a spinning ride that went upside down, and I made a calculated decision to get him on before he saw it in action. Once I pointed out that he'd now been upside down, he was willing to try the coaster.
I'll be the first to admit that he's done things I wouldn't have done until I was 10. He went on X at Magic Mtn at 9! On the other hand, even though 44" was also the height for the wooden coaster at PGA, it was a few more months before he was willing to go on it. Since it's such a monstrosity to begin with, I knew that if I pushed it, it was risking putting him off other rides in the future. I have a video of my friends daughter on that coaster when she was 7-8, and she's screaming "I hate you Mommy!" the whole trip.
Good luck , and have fun. One good thing about MK coasters is that you can work your way up.
Cheers.