AFAIK, and assuming there is nothing to the contrary in his contract, an employee can self-certify sick leave for up to 7 days at a time. After this point you need a certificate from your doctor. However, you have not provided any information on why your brother is on his "final warning" and what action has been taken so far. If he has had a written warning for absenteeism and is continuing to take leave without a certificate if required then the employer is within their rights to discipline him.