The needed actions here are easy to state, if unpleasant to administer. You need to be curt with him. Not too blunt, probably, because that would likely be tiring, like hitting him over and over with a blunt instrument. That's my guess. Keep driving, but end all phone calls because you don't have time. When he tells you something is not funny, you tell him that you found it funny, and still think it's funny (optional: regardless of his opinion.) (If you want to try to be blunt, in an attempt to do him a favor, you can add that you think his sense of humour is lacking. The right answer here is very easy.) Sorry you have to go through with it.