I have every sympathy being an owner of a semi feral who has been here 6 yrs and still cant touch her.
We had a kind of understanding but this is currently gone......sigh
When you break their trust they can be so suspicious for so long and they have a way of being upset that other cats do not have.
I broke trust with Sasa after putting her in a cattery for a few days and then locking the cat flap the first night she was home, she broke out and stayed away for 7 days, coming back with her mate Misa who apparently went to play with her every evening.
To get her to the vet I have to trap her in a corner where she freezes and try to get her to walk into a covered cage,. She again freezes for a time at the vets.
Right now the trust is completely broken and she totally freaks if she is locked in and doors are closed, Think thats why she hates me at the moment cos during the day I am having to keep one door closed.
I obviously do not know your area and how safe it is but I dont know if locking Percy in is a good idea but you know him best. Be prepared for Percy to leave when he is allowed out because I am sure he will punish you if he is anything like Sasa.
I am sure he will come back but could take a while.
Sending best wishes for things going right