There's a couple things i know of you can try if you haven't already, I'm sure others will offer up more. But try to offer prekilled instead of frozen? Buy a live and snap it's neck. Maybe he was used to fresher ft mice. If that doesn't work I would try braining the mouse, people say it makes feeding irresistable. Aside from that I've heard of chicken broth, but I have no clue to its effectiveness.