From one Cooper to another, you may want to ask a friend to come over and bring some treats with them. Put him on leash so he can't run and hide, then let the friend sit on the floor and coax him to them with the treats. Maybe you can make progress before you begin socialization classes.
My Cooper is extremely friendly with everyone he meets but he would originally run and hide. I think it might have been too many people at once or just the fact that people are so much bigger than he was/is that he didn't trust all those big shoes around his tiny feet.