Realistically, you shouldn’t have to ask him that. He should remember to take it down himself if the relationship between you and him is serious enough for him to not still be “looking” for anyone else. Sometimes people forget it is there, but I don’t really see how anyone could forget such a thing. Since it was, of course, apparently such a pressing topic to have wanted to post an ad to begin with; I doubt that one could ‘forget’ that they have an ad up. Instead, they’d look for responses to that ad on usually a daily basis.