You haven't mentioned what type of work he does , but if it is retail it's unlikely he'd get time off now and any sickness absence would undoubtedly be considered very very difficult for the store, however, as you say he has already booked leave over christmas i expect it's not a retail job. Short notice leave in any business creates alot of issues, extra expense if they have to pay someone else to work extra to cover paid sickness absence, pressure on work colleagues who have to fill in and cover the absence, it may clash with other employees planned leave which would double pressure on the management and business. I would suggest that unless the op is essential, I would try to take the date back a couple of months and let work have plenty of notice of the date. In law unless it is jury service he company can refuse for reasons of business needs I believe.