Hi Adrienne,
Thank you for replying. Yes, sorry, by outside I mean in walks. Because I don't want him to just do it on the backyard. For example this week I'm having some reconstruction work being done in the backyard and it's currently inaccessible. Today I've only had accidents in the house, because I go with him on walks and he gets distracted by everyone that passes by (rare this days, due to the lockdown), or by sniffing everything on the floor and (more worrying) eating stones (i live in a semi-rural area, there's a lot little rocks and stones everywhere) and then pees shortly after getting inside.
He's been on a feeding schedule since he's with us and I've been following the troubleshooting ideas on the link you sent above, but without any success so far. He's not afraid of doing it in front of me, as he normally does it with me in the backyard. I always praise and reward this in the backyard, I can't do it on walks as he never does potty on walks.
Best,
Marco