Advice Needed

As you may remember, Campbell was potty-trained last summer.  I am totally confident in him now, and we don't take extra clothes anywhere.  Since we began this potty stuff, we've been putting him in Pull-Ups for night time.  After a few months he decided he wanted to wear underwear, so we tried it.  It didn't work so we went back to Pull-Ups.  It went on like this for quite some time--he'd decide he wanted to try, it might work for a night or two, but eventually we were back in the Pull-Ups.  And that was fine with me, and fine with him.  I definitely never made a big deal out of it--just praised him when he stayed dry.

But recently he started being sort of embarrassed to put his Pull-Up on.  Then he decided he wanted to sleep in underwear all the time.  Now, all this time he's been in Pull-Ups, he would very, very rarely pee in them.  Maybe it gave him some sort of confidence or something.  But now, the problem is, he has the confidence but maybe not the ability.

He started this new thing of sleeping with underwear right before we left for the cruise (more on that eventually), around the first of February.  He did great for about 5 nights in a row, then one accident, the morning before we left.  And then he did great at the grandparents' houses, staying dry in underwear each night.  Then since we've been home, he has an accident about every 3 nights or so.  This morning he made it all the way to 6:00 AM before he had one.  I've talked to him several times about going back to Pull-Ups for a while, but he is adamant about wearing underwear.  I don't know what to do because I HATE washing the sheets and quilt on his bed 2-3x a week, but I don't want to force him to wear Pull-Ups and make him feel like I don't think he can do it.  

So here's where you all come in--I need help.  What do I do?  Do I let him keep having accidents until he finally gets it?  Or do I force him to wear Pull-Ups?  And if I do that, will it hurt his confidence?  Any help would be greatly appreciated!