No, you are under no obligation to do what he’s asked. Employees have a right to request annual leave but you do not have to agree to the request. If an employee becomes sick during a period of annual leave, they have the right to have it converted to sickness rather than holiday; but not vice versa.
You might want to agree a request for leave for a period of time in the future though. This would solve your employee’s cash flow problems and mean that you aren’t faced with a large amount of leave to fit in when he returns.
If you would like to discuss this further, please feel free to contact your local TaxAssist Accountant.