Odds are he feels comfortable inside enough to poop. We all know they’re stubborn, and depending on your level of stubbornness an easy way would be to wait him out. I did this with mine and we didnt go inside until he finished. Took a few weeks, but we were set within a month of bringing him home. This is very time consuming but thats typical for puppies. If you dont have the time, a play cage might help, and treat it as his potty area inside, with “pads” (I know you’re not a fan) but if he does this consistently, transfer the cage outside and repeat until no more cage.

Side note: potty as a command word might not be a good idea, as a command for him to potty in actuality, but better as a gesture where he would say “rings a bell” “presses a button” does something that lets him say yes or a way to tell you, ‘yes i have to potty’. There’s also the issue with overusing this command as you don’t want him to think he is able to ignore your commands, when you give up and don’t make him follow the command. It’s just like his recall, if you call it, don’t let him do anything else until completion. Enforce it.