P.W.
It seems really odd for a boss to ask you to pay for his missed dinner. Usually an employee is reprimanded for mistakes, and if they make too many mistakes they risk being fired.
However, if this job is important to you, you may just have to pay for the dinner to keep peace. Unless I were super-cozy with my boss, I don't think I'd have the guts to oppose such a request.
p.s. - I have to disagree a little with the answers below, having spent many years as an Executive Assistant -- often the Assistant IS expected to remind the executive of such things. That's what an Exec. Asst. is, and reminding the exec is part of the duties of the job.