If he's safely 48 inches with his normal shoes on, he'll likely be fine. But that measurement is with *your* measuring or your doctor's, not with Disney's. I'd suggest you ride this earlier in the morning since everyone "shrinks" during the day. And have your son stand up straight (not against a wall since Disney won't have a wall for him to stand against) and look directly forward, no uplift of the head. If a Cast Member thinks an "almost child" is trying to get away with something, radar can go up. If your son passes the first checkpoint, remind him of he second one and if he is turned away, have a plan in mind for that. There is an ice cream place near Rock N Roller Coaster so take him there if he is turned away.
Also remind your son that the height requirement is there since Rock N Roller Coaster has overhead restraints for safety on that particular attraction.
But, if he does pass both mesurements and rides, hit that ice cream place after! Your son will win - ice cream either way.