Question Sound/volume control

Bit of a weird one this....Our 11yr old,(he has autism.) spends quite a bit of time on his PC, we have time/content/age appropriate restrictions etc but one issue we do have is the fact he HAS to have the volume up loud! (there is nothing wrong with his hearing btw.) No matter how we try to explain it to him or ask him to turn it down, he just sets off in a tantrum every time. I wanted to ask, does anyone know of any software that would let me ''lock'' the sound at a more bearable level and he wouldnt be able to turn it up again because he would need a password or something similar? He is very good with computers so simple things would not fool him. Thanks