If he isn't crying or screaming, I would leave him be!
I know he needs his nap, and hopefully he will figure it out. But I would have a totally different response if he was miserable or keeping the other kids up.
I rarely go in to change a bum during nap time. They all get a fresh diaper immediately before. And once I determine the ones who have a tendency to wake up poopy, I will slather their bums with a barrier cream, just in case. But most of them wake up 5-10 min before nap is done, and poop!