I would also say that using a PullUp is dependent upon how often/regularly it seems to be happening, and what "damage" is being done. There are some really cool pads that you can buy in the PullUp section to put down on the mattress or whatever he is sleeping on. I ended up picking up a package for my 5 year old who, once in a blue moon, sleeps so soundly she pees.
If it's weekly, or if it's upsetting to him, or you find yourself trying to sanitize a mattress far too often, I would put him in one for a nap. I would not make a big fuss, just use the "just in case" reasoning. And definitely cut back on the drink at lunch time to just enough.
I really try not to stress about accidents while sleeping, because all the little people progress so differently. Some are able to do it as soon as they are potty trained, some take years to be able to sleep without peeing. And at least you know they aren't doing it on purpose!
He must be super comfortable at your house, to have such good sleeps. That's a good thing.