Son is autistic/mentally handicapped,and keeps overflowing the bathtub.He is 9 yrs old& i need a safety.?

he is so strong,that the typical safety device that suction cups to the wall will not work,because he will be able to tear it right off the wall! our overflow valve leaks like crazy,and we can't afford to fix it just yet.it is causing much damage to our house.even when we fix the overflow valve,we don't want the have the water running anyway,wasting hot water . we don't want to have to go down to the basement everytime we give him a bath,to shut the other water valves off.