Hey L.,
A few thoughts:
1. You didn't mention how long it's been since your son has been wearing underwear to bed- he may just need more time in them to feel when he has wet himself. My son is 4 1/2 and probably had accidents at night ten times over several months before he stayed dry. He went from diapers to underwear at night.
2. If you feel it's taking longer than other kids you know to stay dry, call his pediatrician and talk it over. My older brother wet the bed until he was eight. I've heard it's not uncommon for some kids' neurological wiring to develop to allow them to stay dry through the night until 6,,7,8 yrs old.
3. Could it be regression? When young kids are under emotional stress (new sib, moving, divorce, etc.) they can regress in different developmental areas. If you don't think stress is a factor, try not to create stress by making a big deal of it. Though having to change sheets and a soaking, whiny child in the wee hours does take a toll!
4. Instead of one mattress cover and sheet try two: mattress cover, sheet, another mattress cover, another sheet. That way, when you pull the top sheet and mattress cover off, you'll have dry ones underneath.
5. I switched from a plastic to a padded mattress pad because the plastic was cold in the winter and I didn't want to contribute to any accidents!
Good luck!
D.