Help with dog potty training?

my dog is not potty trained no matter what we do. He is a 3 years old shitzu, we adopted him when he was 1 1/2 years old from a shelter. We take him out at least 3 times a day, he pees and poop outside but if we leave him in the house alone he peeps and poops anywhere. We've tried crating him in one of those wire crates, he still goes in there. We adjusted the size of the crate so he wont have much room to move around but he still goes in there. We have him in a small travel crate for when we go out for a couple of hours and he doesn't go there but when my husband start work and he will be home for at least 8hrs i can't leave him in that small space for that long. We tried punishing him but he just gets scared of us to the point that if we raise our voice at him he pees on himself. He doesn't even indicate when he has to go which makes it even harder. Don't want to let him go because of my kids but my husband and i are tired of him especially sense we have a house being built and its 95% carpet. Any idea on what to do would be very help full.