It's possible that he was never taught what to do with it by his mother. You may just have to get down there and eat that mouse in front of him... unless you can get a tin of cat food that looks and tastes like mouse.

The run and catch instinct is always there, it's the 'this bit is safe to eat' and 'only eat 3 legs because the big people love finding the 4th leg in their shoes' lessons that have been missed.

LTS