If your going to try to feed him rats your going to have to be persistent for a few weeks. I would get the smallest rat available, that way it wont be as intimidating for him. If he doesn't take a rat the 1st week, wait another, if not wait another. I wouldn't go for longer than a month without him feeding.

Another option is to feed him multiple mice. Thats what I did. For some time I though my female didn't want rats but I found out the only reason she didn't want them was because she was just coming off winter fast. Now that shes been eating for a few months she will take anything I put in her tub.

There is absolutely nothing wrong with feeding multiple mice vs. a rat. The ONLY downside is expense. If thats not an issue, don't stress.