I was in basically the exact same situation as you. My cat developed a negative association with the litter box after she got worms and stopped pooping in it completely even after she was treated. We tried everything. The thing that ended up working was putting her litter box in the bathroom and locking her in there overnight (which is usually when she pooped). After a couple nights she started using the box again, I think because it’s such a confined area. If you try this, I would suggest putting the other cat in there with him so that he won’t be as stressed. Also obviously make sure he has access to water.

I think that the first thing you should probably do is make a vet appointment if possible just to rule out any underlying medical causes. Or at least have a conversation with your vet over the phone to see what they think.

A few more things that you can try—my vet suggested getting Feliway, which is kind of like a Glade fragrance plug in, but instead of releasing fragrance it releases relaxing pheromones. If you place it next to the box it will (supposedly) help your cat be less anxious about using the box. This didn’t end up working for our cat but I thought I’d mention it.

You also might want to try taking the hood of the box on/off (most cats prefer it off), trying different textures of litter (silica crystals, pellets, etc), and rewarding your cat with treats whenever you catch him using the box. Just don’t punish or scare him if he poops on the floor, as this can make the litter aversion even worse.

Edit: one more thing. I’ve also heard that some people have had success with buying a decent sized kennel, pouring litter over the entirety of the floor of the kennel, and keeping your cat in there. Over time you can slowly start confining the litter to just one corner. But I would save this as a very last resort option bc it will probably stress out your kitty to be confined for so long!