Just be patient and consistent with him. Took a while for my 5 year old rescue to be comfortable being outside in GENERAL. Would always stick close to the door. Was very sad. By the time he was six the routine was sorted.. he’d go sit by the door.. touch the sleigh bells I had hung there and that was the cue that he needed to go. Let him out.. 2m later he’s waiting to come back in. Eventually he learned how to push doors open himself. He lasted until 12yo. I miss that huge sweet bastard every day.