Not a very nice family member! He obviously told the Cuncil he no longer lived there & gave your son's name. However, the Council would then have sent your son a bill before issuing a Court summons. He should talk to the Council about what exactly happened, & make an arrangement with them to pay the debt over a period of time. If he does this before the Court date, there is no need for him to go to Court. If he gets the Council to admit they did not send him a bill then he should ask them to withdraw the Court summons & cancel the Court fee (which he will otherwise have to pay in addition to the tax).
If he lives alone, he should make sure he is getting the 25% discount.
The landlord cannot remain on the electoral roll at that address if he is not living there - he should be on the roll at his present address.