Hmm...I'm assuming here you're not interested in him?
If not, if he does it again, perhaps just mention that you don't think that's appropriate work place behaviour. of course, after that if he suggests something about out of work place behaviour, you know he's definately interested in you so you have to nip it in the bud by telling him you're not interested.
He is your boss of course and one wold hope he's not trying to abuse his position of power - just that he's a chap who is interested in you (he's only human after all!!!). Of course, if he does try and use his authority to continue this behaviour, you might need to make a stand.
If you have an HR or a director above your boss's ranking, definately mention it to them at this point.
But I would try, in the first instance, to deal with it as discreetly as possible.