Welcome to PT!
how old is your cocker?? did you get him from a reputable breeder? there are lots of cockers that are bred by ppl who don't know how to breed properly and the pups will have behaviour problems. it could just be that he wasn't socialized well enough. never scold him for being afraid. dogs can react aggresively when they are afraid. i agree with the suggestions everyone said.

introduce your dog to knew things. do not force him into it. give him lots of treats, reassurence, and praise when he is being brave. Good Luck.