My DD has had the occasional dry diaper. Usually this is when she's slept pretty soundly through the night and I wake her in the morning rather than the other way 'round, or at least get up promptly when she wakes up. Most babies, I think, pee when they wake up, not actually while they're asleep. In the middle of the night when my DD wakes I don't change her, just nurse her back to sleep. So she probably pees then, and I'm slow to get out of bed in the morning, and she often pees before I get out of bed and we change her out of the nighttime diaper.
Did he pee shortly after waking? When my DD has gotten up w/ a dry diaper and I've put her on the potty, a big pee has almost always been the result.
If he slept soundly last night and peed this morning, I wouldn't worry about it at all.