Mice. Try frozen/thawed, and if that does not work, try a live one, but watch very carefully and supervise closely. Do NOT leave a live rodent unattended with your snake. If the snake is not in the mood to eat, the rodent can do damage. Get some frozen mice that are about as big around as your snakes thickest part.
If your snake does not eat for you in a week or so, please release him where you found him. Also, if he won't settle down for you and allow you to handle him after regular, patient attempts, remember, that a captive bred ratsnake is not expensive, and will make a potentially better long term pet.
Also, don't handle him for a couple of days after he eats...he might puke. Other than that, try to get him to let you handle him without biting and/or musking eventually. Some wc will calm down for you...others won't.
Finally, what do you mean by "keep him outside" for awhile? Make sure that he is not in a tank in the sun. A sunlit tank will heat up quickly and may kill the snake. Good luck, and remember...if this one does not work out, there are lots of cb ratsnakes out there for pretty reasonable cost. Duffy