I wouldn't switch to gerbils. Instead, try another rat but make it a live rat pup. That should be easier to get down. Try it at night, and leave him alone in a dark room after you place it in the cage. You should be able to leave it in overnight without worry.

There's not much you can do to increase his appetite besides the vitamin shots. You can try to get him in feeding mode before offering the rat pup, but it doesn't sound like that's necessary. Oh, and the reason I don't suggest switching to gerbils is because your snake is already imprinted on rats and switching food types around may just add to his stress.

Good luck!