I can tell you from personal experience that Disney is very strict about height measurements. I remember one time that my daughter was measured 5 times by 5 different CM’s before we got on to Mission Space! We had to laugh about it, because she kept telling the CM, that she just rode it yesterday and she didn’t shrink! However, don’t be so quick in thinking that you son won’t make the height requirement. Measure him at the ride before you get in line. We had one experience when my oldest daughter measured up for Star Tours, but didn’t make the cut for Body Wars (she was wearing the same shoes for both). We also had a experience in which we measured our daughter at home, thinking she would not make the Rockin Roller Coaster, but just a few days later at Disney, she make the measurement by at least a ½ inch.
BTW, my daughter rode Dinosaur when she was about 4 1/2 and she thought it was fun! I think kids that age have less fear than older kids.