Don't let him out. Ignore his cries.
It'll be hard at first, but he'll eventually stop.

>>2127091

How long can that be expected to take? Is there anything we can do to aid the process? We've tried it in the past and even after a week or more of ignoring it, he remains relentless about it.

I've read about the idea of disguising one of us outside and spraying him with water while scaring him as he stands near the door. It seems like that's nigh-abusive, any opinions?

>>2127095
You just need to stick with it and he'll forget the outdoors eventually. You can try some calming products like Feliway to see if that helps as well.

You should buy him a kitty leash and take him on walks whenever you can. The leash will probably be a failsafe if anything, as most cats refuse to walk and would rather be carried. You just need something to hold onto if the cat tries to flee

Harness train him (look up guides, DO NOT just throw a harness on him. They aren't dogs and they need different training and a different H shaped harness)

Play with him inside until he's panting and laying down

Look up catios (outside cages for cats, usually attached to a window)

>>2127095
My cat used to be terrified of the outside, then she went out for a bit and loved it, then we had to keep her inside and she eventually stopped caring about the outside. Cats can adapt just fine with enough time.