Professional life =/= personal life.

You don't have to like everyone you work with, nor do you have to act all chummy chummy towards them, but when it comes to doing your job, you should just do your job. Presumably it's part of your contract to try to cover shifts if a fellow staff member is ill or something, but if you can't (be arsed), then you don't have to. I'd just tell him you're busy as a mothersmurfer if you don't want to do any extra hours. xD