Hi Julie, I'm Linda, a vet wirth 10 years of clinical experience.
I'm so sorry to hear this and can imagine it is hugely stressful for all involved.
Some cats do not get on well with others and are more ntaural 'lone rangers'. They get stressed by sharing their territory and are not happy to co-habit. This may well be the case with your boy.
As well as the calming products/medicine you use, the aim is to create a place where all cats feel comfortable.
This usually means lots of space and resources of their own. So each cat should have their own litter trays, water bowls, toys, scratching post, cat trees etc. and nothing should be shared.
They should also have their own hiding spaces and places to sleep.
This goes a long way towards reducing conflict.
In a situation like this where the issue has been ongoing, I'd consider bringing in a feline behaviourist to assess each cat and make a more tailored plan to help them co-exist.
In some cases, we will ultimately need to re-home the cat if they are stressed and making the other cats miserable. I appreciate this is likely not what you want to do, but is sometimes best for all when all other avenues have been exhausted.