My DS is working for a guy who has a delivery franchise of one of the big companies. So he buys a van and gets lads to deliver and pays them a flat rate. The lads are self employed. DS has had a bit of a hard time and told his boss he needs some time off cos he’s struggling mentally. He says no. So DS said he’s leaving. Goes to work his first day notice but has an episode so turns back with the van. Boss is now threatening to take all his pay off him for shifts not worked and the notice which he can’t work due to mental health. Can he say he’s sick and get Drs note for second week of notice period?