He didn't reply.
Dear Mr Potter,
We are pleased to inform you that you have a place at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment.
Term begins on 1 September. We await your owl by no later than 31 July.
Yours sincerely,
Minerva McGonagall
Deputy Headmistress
Quite why they started mithering him before the deadline is another question, possibly because they normally get responses back earlier?