Hi,
I don't think your son has to be there, but for a Spanish school you have to have the 'empadronamiento' certificate (i.e. a registration certificate from the town hall). In order to get it you need some proof that you live in Benidorm (a contract with your name and Benidorm address on it). To avoid problems, I'd advise you to get a Spanish speaking person to go with you and get all the information you need and get your son to learn Spanish NOW.
Good luck!