Have read your situation and while I don't have feral experience one thing that popped in my head is in the eventual catio (as big as you can make it) add some vegetation, shrubbery, ornamental grass than he can hide in outdoors- since he hides in shrubs at your place now.
Cat grass growing also? I have one cat that loves it. Although he chews it to nubs in the pot so I have to just give it sometimes in the pot.
Find old logs on the ground to put in? Leaning up to climb? I've never done a catio either but I'd think that would be good to do. Scratching surfaces outdoors.
This is hard. But love for Oscar can win out.
Cat grass growing also? I have one cat that loves it. Although he chews it to nubs in the pot so I have to just give it sometimes in the pot.
Find old logs on the ground to put in? Leaning up to climb? I've never done a catio either but I'd think that would be good to do. Scratching surfaces outdoors.
This is hard. But love for Oscar can win out.