Gerber makes a plastic liner to go over training pants or cloth diapers. You may see if their biggest size would fit over your son's skivvies. It may help contain the pee at night (but then again, if he pees alot, it may not do much good either).

Some kids just really have a hard time at night. We had a friend stay the night once who was 8 and still having some problems at night. She wore GoodNights just at night (my DD didn't and still doesn't know this. The little girl's mother and I kept this very quiet so it wouldn't "leak" out at school. hehehe, pardon the pun!). The bladder control just wasn't good enough while sleeping. Pullups or goodnights (or generic versions of either) only at night may be cheaper/less time consuming in the long run than the expense and time taken to change the sheets and blankets and shampooing the sofa (or ruining the sofa). Just my 2 cents.