I agree with Matt L
Your cornsnake is acting pretty normal for some baby corns. They get scared by the pinkie and it's perfectly fine if it is eating just heads, because if its eating heads I can promise you in no time it will accept the whole pinkie!!!
Kepp feeding the heads and trying whole pinkies. You can try feeding a head then right away after its entirely in the mouth and a little down the throat offer a whole pinkie. Sometimes the feeding response they have will just get them to eat another item right away without really caring.
good luck
bmm