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.