S.R.
There are a few things you can do...
Personally, I would invest in a removable pad for him to lie on as he's sleeping, that way, you only have the pad to wash and not the entire bed. Also, encourage your son to limit his fluid intake after a certain time in the evening, and be sure he uses the bathroom before bedtime.
With this being a type of medical issue, I would also encourage you to consult a pediatrician and see if this might have some other cause that needs to be addressed.
I really hope that it gets better!