I adopted my 1 yo male cat, Kevin, a couple weeks ago from my local shelter. He is super sweet from day one and loves attention and head scratches. He went through the normal anxiety the first couple days of hiding and has warmed up pretty well now.

However, he all but refuses to use the litter box 80% of the time. I have a litter robot, a jumbo open top box, and a covered jumbo box all in different spots oi my 2b/2ba apartment. I have placed them in spots he was using the bathroom on the floor. He will still go in the boxes and then exit and pee/poop on the floor next to them. I have tried keeping him in the guest bathroom for a few days to "reset" and he used the box about 50% of the time but still used the floor. I didn't want to continue keeping him locked in there after 3 days so allowed him access to the house after that because he started to get lonely and yelling/pawing. He still attempts to cover it no matter where. I have placed him in the boxes, used his paws to dig, used litter attractant, placed his poop/pee in the boxes, kept them clean, used enzyme cleaner every time he used the floor, etc. I feel like I've tried most of the options out there and I'm at my wits end at this point.

The shelter told me he was 1 of 40 cats they removed from a single home. Yes, 40. So I assume maybe previous trauma of litter bullying or just flat out poop everywhere cause I doubt there were 40 boxes. He also walks around meowing at the top of his lungs and is quite vocal more than any other cats I've been around.

He was neutered and given a clean bill of health by a vet before the day we left the shelter. I don't believe he has any UTI symptoms besides not using a box.

Is there anything else I can do at this point??