My dad has suffered from a bad back for a long time, he has curvature of the spine but it doesn't usually cause him problems. During work he lifted heavy fridges which is not part of his job as he is a delivery driver but he is constantly expected to unload and reload vans. Due to this his back cramped up and he was stuck on the floor for hours while I got him medication from the doctors. He was off for 3 days (monday-wednesday) and then on Thursday rang his boss and asked for his van back. His boss then told him he no longer had a job as he didn't think he was fit to work. He doesn't usually have days off sick, maybe more than the average person but not excessively. Is this lawful?
Oh and not only was his movement too limited to drive but the medication prescribed was so strong it would not have been safe for him to be driving a van. I'm looking for answers regarding english law.

Covered by WC where he works? Were they aware of his spine curvature? Sounds like it should have been a Workers Comp claim and they can't fire you for that.