Just get him a mouth protector (like the kind you wear for football) and have him use that at night. I think they might make special ones for night time use that stay in better, but I never looked into it.
And the reason he doesnt like brushing his teeth is because it is PAINFUL! You said it yourself, he wore his enamel down, so his nerves are exposed. Also, most toothpaste has this ultra-minty flavor which is bad enough, but on raw nerves it is like getting stabbed with a knife. Get him some sensodyne (pain numbing) tooth paste and see if that helps.