Advice

Got a new aussie, hes 11 months, about to be a year old. he was raised as an outdoor dog and would just come home for dinner and to sleep. we brought him him home 4 days ago and he wont eat the food we got him, has no interest in any treats, wont walk on a leash(we had a collar on him but he escaped, a harness was the only option as we have no fence and want him to be able to come outside). He will take a few steps on the leash then refuse to move and if i try to prompt him hell just sit down, and i practically have to carry him back in because well be out there for an hour just sitting and he wont walk to follow me back. Dont even get me started on training him because theres nothing to reward progress, he wont eat the treats! I love this guy and I just want to be able to get him used to all these things so he can live good. Does anyone have any tips?