it's really normal for kids to wet the bed. I dont think it's a deliberate act at 3.5yrs, but just normal night wetting. Does he normally wear pull ups? Can you use brolly sheets as well, or hospital blue pads, so you don't have to wash all the sheets.

Night time continence is largely a hormonal thing, and some kids need lots of time (I have a friend that used to periodically wet the bed until they were 12).