My daughter is 6 and REALLY wanted to ride Rock N Roller Coaster this trip. She was 5 1/2 last March, and they wouldn't even let her in the building to see the preshow. We measured her and she was only 47 1/4 inches in her bare feet, I think. We had her wear a pair of fairly thick soled shoes - but nothing outrageous. These were the shoes she regularly wears anyway. I also told her she would have to stand up tall when they measured her, as I was sure she would be measured. They measured her outside, but the CM's were laughing so hard at the way she was stretching her neck, I don't think they even checked the stick, but did warn us she would be measured again inside. When we got to the loading zone, the CM measured her, and I'm pretty sure she was just a hair too short, but she was so confident that she was tall enough, they said she was all set! She rode the ride, and LOVED it!
Just be sure that your son has on a pair of fairly thick soled sneakers, and I'm sure he won't have any trouble.
Good Luck!