Hi,
Are you sure he isn't just not hungry and defensive striking at it to make it go away and leave him alone?
When you changed him onto rats what size are they and what size is he?
Could you also describe the feeding routine you go through? Are you taking the rat out if he doesn't eat it within 20 minutes and then leaving him in peace for 7 days and trying again?
Has anything else changed husbandry wise lately? When did he last sucessfully eat for you?
If you can give us this info it might help pin down the problem. Also do you have a cage set up for the rat as leaving him in the same tank could obviously lead to some extremely serious problems.
dr del