Pick out a healthy crawler rat that is about the same size as a mouse, and rub it thoroughly in mouse pee. (Mice usually use the corners of their cage as a latrine, so find the wet spot, and rub it in good).
It's gross, but this heavy scenting usually does the trick.

If it doesn't, then use a little 'reverse psychology'...rub his next mouse in soiled rat bedding. If he takes that, then try it the other way around again.