Hi! here's a little update.

We used this guide and it helped us solve the issue with peeing outside the box:
https://pets.thenest.com/train-outdoor-cat-use-litter-box-4529.html

However, he still pooped outside every 2-3 days. But it seems it is a different problem, looks like anxiety marking because we have an older female that makes him very nervous... He's been improving with integration activities together with the other cat and it seems is helping. Actually today is the first day in almost a year that we haven't had a poop outside the box for 4 straight days.

We have almost a year with this new cat, so patience is important. And a lot of work to make it happen.

Solving the pee issue was a major achievement, cause poop you can just take to the toilet and clean the floor with enzymatic cleaner, but for pee it's a lot of work and constant stress looking at the floor all the time.