It's hard to know the "why" of such a situation, but you can start now getting him accustomed to the outside again, just like you would with an unknown dog, or a rescue pup.
Take him outside on a leash in the morning and after meals, and be prepared to stay outside until he does something. Go for a walk on leash, and keep him on the leash in the yard so he cannot just run for the porch. Be prepared to stay a while, and when he does "his business," praise, praise and praise.
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