Rewilding a sick cat?

Hi all! I'm new here but desperate for some advice, thought l'd give this a try! I'm a big cat lover and have had marvellous cats all my life. 2 years ago I set a have a heart trap for a cat l had heard yowling outside for three nights and found I had captured a ten week old kitten. But he was a scaredy cat. I did everything I could to make him comfortable but had to accept that he might not be super friendly. When he was 6 months old, though, he started running around the house and pooping very hard poops (he had never had an issue with not using the litter box before). We took him to the vet, tried all sorts of diets and medicines for both constipation and anxiety, and nothing worked. For two and a half years we've been trying to figure out what to do, cleaning up after him practically every morning. A behavior specialist we hired said the only remaining humane option is euthanasia. I disagree, but the rest of my family is coming around to the idea. Does anyone have advice, experience, anything that could help? I'm tempted to try and let him be an outdoor cat but the times he's escaped he's acted very afraid, and the behavior specialist said it would be cruel to let him outside because he'd be scared Thank you if you got this far, I would be so grateful for anything anyone advice.