Sometimes kids get really, really tired and start to sleep through their bladders. Has he been really busy the past week?
Our son gets very stressed, and he has issues with this. If there's been a change, or a stressful event, he'll sometimes wet himself a bit.
On a side note: an easy changing item is to buy a protective sheet (we have one that is nice---it doesn't feel/smell like vynil) and we make the bed twice. Make it once, and then put the second sheet protector down, and make it again. Then, in the middle of the night, you don't have to remake the bed again. Just pull the top set off, change the boy, and off you go. The next day, slip a new set on, making sure the bottom set is dry. It's SO HANDY in the middle of the night.