Few more things to try:

1/Stick him in a small rub with food and leave him overnight, my baby takes it near enough instantly in a tiny rub where anything bigger it would prefer to cruise around in.

2/Rub the mouse in either tuna or chicken fat to "spice" it up a bit.

3/Make sure the mouse is warm when attempting to first feed.

Braining is supposed to be a good way to get a response but you have tried that. I am by no means an expert but these are the things i have read in my short time owning a corn snake.