Just another note, it is really ok for a 5-year-old to still wear pull-ups to bed. With my son, I decided I would start putting him in underwear at night when he started consistently waking up with a dry diaper. I expected it to be when he was 5 or 6 but he surprised me and started doing it about a month ago (he's 3 1/2). I waited until he'd been waking up dry for a couple of weeks straight, then I switched him to underwear at night and he's been doing great! So my point is, maybe you should hold off on night training until your child is already waking up dry. To help him get there, try not to let him drink anything within an hour of bedtime and make him go pee right before he gets into bed.
If you really want to go forward with the training, though, I second the idea of the plastic mattress cover. I keep one on my son's bed in case he does have an accident.