Dog Stopped Using His Litterbox?

I have an 8 month old Chihuahua who has never been housebroken. We want to train him to use a litterbox with potty pads inside. First we trained him to use the pads. He did pretty well with it with not very many accidents after a while. Then we moved the pads into the litterbox (which we already got him used to).

He did very well entering on his own and using the litterbox for about 2 days. Then he stopped using the box and pads altogether. Now he will hold it all day until we let him out of the washroom and uses the bathroom all over the house in places he never did before. What are we doing wrong?

By Ashley