Hi and welcome ll3ctre
I take it he has a valid British passport? If so and his passport is endorsed as a British Citizen then he does not need a visa for stays up to 90 days.
If the person has a British passport but is not
an endorsed British citizen living in the UK or a British Dependent Territories Citizen, or British Overseas Territories citizen then they are only allowed to stay for 30 days without a visa
I hope this helps