Is expiation binding upon one who swore to expel a person from his shop if they brought people to play, then was surprised to find the very person they had forbidden entering the shop?

If the boy did not enter by this person's choice and consent, then you are not required to expel the boy or offer an expiation for breaking an oath. This is because the oath was contingent upon him (the person) letting him (the boy) in, and he did not let him in.