well... if you do this now, get a bunk bed with very effective sides, and make sure you have carpet and good padding under the bed ;). I think all three of my kids-who-have-been-a-top-bunk-kid have at some point rolled off their top bunk once (or, in one case, twice). All of them were 4 or 5 ish I think ... currently we have all four bunkable beds with feet on the ground ... .
We started in bunks around 4 when I was a kid. I rolled out of my bunk while sleeping twice that I can remember, I think about age 6 and age 9 ... so, what're gonna do ;)??
Falling off a bunk is pretty disturbing, but generally the kid lands pretty flat.
I agree with the previous answer, that the ladder situation is really the most dangerous aspect--on all the beds I've ever been around.
The main thing I'd say, is does your son have a very good idea of where his body is in space and is he generally coordinated? There are LOTS of kids who can't manage a bunk safely until about 6 or 7 because they haven't developed these abilities yet ... (hence, I had one who fell off TWICE in one year, before we moved him back down to a lower bunk).