I just keep asking every day and eventually he gets so sick of hearing it that he gives in. I make sure that when I ask I'm doing it in a light-hearted, joking kind of way so that I don't put too much pressure on him. It's worked 2 yrs in a row for me. I've already started asking for my 3rd Yorkie a year in advance.
In all seriousness, if there is a REALLY good reason not to get the Yorkie, then you'll have to agree with him, but if not...I'd just do it anyways. I'm a bad influence though, don't listen to me