Pruners, you say he doesn't know to be embarrassed ... he shouldn't have to feel embarrassed. It's very much within the normal range to not be dry at night at his age; something like one in four 4-year-olds and one in five 5-year-olds aren't.
Leaving him bare-bottomed may work; equally it may not. My daughter was desperate to be dry at night, but leaving her bare-bottomed (and being wet three times a night) just left her upset and feeling wretched. We layered the bed, which made things a little easier for us, but she was simply not capable of being dry, no matter how much she wanted to be. We let it be her choice when she tried, but each time her distress meant that she couldn't carry on with it and went back to PJ pants within a few days.
The age at which a child is dry at night is partly genetic, so the fact that his uncle wasn't dry until late may well be a factor. If either you or your dh, or even any of your parents, was late being dry, that adds to the likelihood.