Have you considered placing puss puss in a crate while you’re not home? Some people think they are cruel, but some animals need them to feel safe and secure. Worth a shot

Aww this would be hard to see. Have you tried putting treat toys around that need to be hit to access? My cat loves those. Also putting out cat tunnels to run through, or a laser toy that runs for a few minutes. We would turn it on just as we walk out the door as a distraction

Awww! Poor kitty! I know how hard that can be. We have an anxious pup. Some things have already been suggested like the possibility of you getting another cat? Or creating a crate or cosy little cage that he might feel safe to go and cuddle up in. Try adding a peice of your old clothing to it so he can smell you and have some comfort. You could also try leaving some music on or the tv on for some background noise.

We foster kittens and when we have a new cat or kitten coming into our house we give our more anxious cat an anxiety collar. It has really helped her to adapt to changes and may also help with separation anxiety.
When we had one cat she was heavily reliant on interactions and talked to birds and bugs. We ended up getting a second cat and their relationship is beautiful. After some teething and introductions then now cuddle at night and groom each other.

Have tou tried crate training? Some pets feel more secure when shut in a small space

Our local vet has a plug in device that emits some kind of calming pheromones. There is also collars available that do the sane thing

aaww….poor Darling.
Im wondering if perhaps he needs a friend. Perhaps see if you could introduce another kitty into the mix. They might not get on to start with but it wont take long and they will become friends. Perhaps that will solve the problem as then he would never be alone.