Hi Axel Miller!
You may want to consider saving some of your poop/pee (if you can) and transplanting it outside in the spot you want him to do his business. That way he'll sniff the area, and recognize his scent and want to potty there? I lived next to a dog park and
training Echo to go there was so easy probably because all the would sniff was pee EVERYWHERE! =D
As for
training inside the house. Instead of just a caged off area where you can leave him to his faculties. Perhaps you should leash him in the house so he's stuck being around you at all times. He might be too timid to pee/poop near you OR...if he does, you'll be closer to quickly correct him and pick him up and take him outside to do the business. As always always give a treat for going outside! =D
Your other alternative is to do puppy pads if he's only comfortable going inside and then slowly moving the pads towards the door and transitioning outside. But that's just an extra couple of weeks of
training that you really don't want.
Good luck!