depends on how much is earned. Pensioners have a tax free allowance like everyone else. He would pay tax depending on total income. The hours are irrelevant. If that is the only income then it is unlikely that tax would be due, unless it's �1000 per hour!
He can earn �6830 in the current tax year without being liable to tax. Pensions, including state pension are taxable and count toward total income in the year, as do interest on savings and dividends from shares. In the tax year in which he turns 75, the tax free allowance goes up to �6950 (current year's figure).