Chloe only uses her pee pad. I make sure to use a favorite treat that she only gets for doing that.
Since you want to train him outside maybe find a treat he loves and only give it to him when he pees/poos outside. Make sure to take the treat out with you so he can get it right away , the second he is done , so he knows he is getting it for going potty outside. If you bring him in then give the treat , he thinks he's getting it for coming in the house and may rush to get back in ..( you wouldn't believe how long it took me to figure that one out

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Our other dog Syd Vicious ...aka Sydney ( not a Yorkie) goes outside. And that's what I did with her. It took about a year to get her trained to the point I can trust her and take my eyes off her while she walks around the house ( just our main floor , the stairs are gated) . She will hold it all day if she had to.
I just remained as consistent as I could and didn't let the ( many) set backs get me down. I know it's hard but they will get it. I did eventually cut back then cut out the treats for peeing outside , I just gave her lots of praise and she responded just as well. But I never stopped giving her the treats for pooing outside. She is 3 now and I will continue it , because she likes it and tries hard for us , and well , it works