"thin plaster over somekind of cardboard"

Are you sure this isn't drywall? If not I have never heard of plaster over cardboard.

Bless you Grandma barbara2007. Have you talked to his doctors and couselors? If not you need to, they will try to help you focus his upset onto hitting a pillow or something before he hurts himself.

Get a board narrower than the hole top to bottom and wider side to side. Put a large screw in the center of the board to hang on to. Slide the board in the hole and on each side place two or more drywall screws to hold the board in place. Remove the large screw. Cut a piece of drywall to fit the hole and use drywall screws to fasten in place. A thiner piece than the wall will work. Get a bag of 75 minute drywall powder and mix up less than you need. After a few hours do again. use drywall knives to spread the mud and smoothen it.

Keep the open bag of drywall stuff closed and off a concrete floor. Use as needed.

$20 or $30 should get you all you need to do a lot of holes. This includes the tools.

Lumberyards and home centers do sell broken pieces of drywall, cost is 1 or 2 bucks.