Dollar stores sell the vinyl tablecloths and the normal 50x70" size can easily be used folded for extra strength.

What we have on the couch for our special needs daughter cause let's face it adult tena briefs suck big time but are cheaper than the diapers so prefer to do a tub of laundry every few days instead. From fabricland we bought a piece of thick vinyl that is sort of a damask floral pattern on one side and a bit fuzzy on the other to give you an idea what it looks like. It is meant to go on your good diningroom table underneath a cloth tablecloth to protect the wood from water stains. It is a bit expensive but works great. We even bought another piece of it to go on her bed. It is the width of a double bed mattress so you would only need to buy the width of bed you want for him to have.

Also find out if he is sleeping well at home. It could be that he is so exhausted he is going into a deep sleep.