1) Could it be due ot the change in food?
Yes
2) How often should I offer him food? Every nigh till he eats, or every second night? or so on.
Wait one week before offering again, if you offer too soon or too often after a refusal it will only stress your BP, leading to more refusal.
3) Are their any tricks I can try to get him to eat these mice till the store gets some rats in?
The problem is that even though you might manage to switch him, he might never go back to rats afterward, so make sure this is really what you want.

Maybe it's time for you to find a reliable source of F/T rats or order bulk.