As the driver, you are responsible for the safety of the vehicle whilst it's on the road and you are the one who would be prosecuted if it caused an accident due its being unroadworthy. However I'm not sure sure whether a faulty clutch represents any actual danger.
Arm yourself with warm clothing and survival gear and go out on your round tomorrow. If it breaks down, phone him and tell him so, and ask him to get you towed back. If he refuses, then lock it up and get yourself home via taxi - making sure he gets the bill, of course. If you're on the motorway when it breaks down, call the police and tell them you're stranded. You won't be in trouble with them. Your boss, however ...