It is very safe, with a padded, over-the-shoulder restraint system that is separate for each rider, and offers a much smoother ride than many other WDW coasters, such as Expedition Everest or Space Mountain -- but in my opinion, RnR is MUCH faster, louder and more intense than the other coasters, and would be especially so for a little guy who is just reaching the height requirement (and therefore hasn't been on many coasters before.)
RnR rockets out of its starting point and goes very fast in the dark, with Aerosmith screaming [gorgeously, if you're a fan, which I am] loudly in your ear the entire time. It includes three corkscrew inversions (turning upside down). My son insisted on trying it out on his first trip to WDW. He was 6 years old and 49" tall at the time, and the most intense thing he'd ridden at that point was Big Thunder Mountain Railroad. Although my son ended up saying he liked it (with the caveat that "those Smith guys' music is AWFUL!"), it was only after several minutes of stunned silence upon exiting the attraction that he came to that conclusion, and he didn't ask to go on it again. While he's a daredevil, I think RnR, for him, rode the fine line between, "hair-raising excitement" and "horrific psychological trauma." (Still, totally safe -- the restraint system had no trouble keeping my son from bouncing around, even though he just met the height requirement and is super-skinny.)
If you want to let your grandson work up to it gradually (if he hasn't ridden rollercoasters before), I'd suggest a multi-step introdoctory program, beginning with Big Thunder Mountain (to see if he's okay with the general idea) and moving onto Space Mountain (to see if he can handle coasters in the dark); before trying RnR. In the alternative, if he can do Expedition Everest, then he is fully qualified to move directly on to RnR.