Would like to know age please?
Also, where did you get him at? I know that Pet Stores feed mice and depending on his age he may only eat mice. You said you had tried rat. I would try a mouse. I have 2 snakes that will only eat mice. After a snake has only eaten mice for its whole life, the snake's only recognizes mice as food not rats. I start mine off on mice and switch to rats asap, but some chose to stay on mice.
My advice would be, try mice. Maybe that is what he is used to. After that do the opposite of whatever you are doing.
For instance if you are trying to feed live, try to feed FT instead. If you are trying to feed male, then try female instead.
I had another ball that would only eat dark male mice. No white, no cream, no female. It was crazy.
Good luck.