So if I understand correctly he was being fed live and is now being offered f/t.

What temp is the food item when you feed it and how are you measuring its temp.

If you think about how "heat vision" works a live rat (hotter towards the head and center mass heat) is going to look very different from a f/t rat (possibly uniform heat but superficial) so he is probably having a little trouble recognizing it for what it is. Put that together with live rats that it is used to seeing on the ground and f/t that are apparently falling from the sky.

Your answers to the 2 questions above may change this but I would suggest raising the heat on the food item and be sure to make the front end warmer than the back end.