Be very careful how you socialise a dog this scared. Remember as a cocker he is quite small, so most other dogs will be bigger than him. Choose a class very carefully, ideally you want other dogs on lead under control, while yours is free to go round and approach and socialise with the other pups, unfortunately you would need a very good trainer to know this. In a situation where all pups are let off together to "socialise" will be a nightmare for your pup and will just compound his fear, at the moment he is using flight to avoid the things he is scared of (other dogs), you don't want him switching to fight.
it is worth trying to find friendly dogs in your area, go to the dog walk on your own and talk to a few people, see if you can find some well trained dogs that will lie down whilst your pup approaches.
If you get this right, you will buil;d his confidence around other dogs. In my experience cockers are little squealers, I once had a puppy class with two cockers in who if another pup so much as glanced their way, they squealed, i took it carefully and slowly, by week five they were leading the other pups a merry dance.