I hate to suggest adding to your work load, especially when you are doing so much already to help ferals, but I also hope that you use a large cage. As you know, confinement is part of the routine to socialize feral cats and a week of this "treatment" will show you whether you should release him with a "fare thee well, we helped you as much as possible", or consider that with foster care he could turn into an adoptable cat. I feel guilty even suggesting this since I know the numbers of cats needing homes are incredibly high and you can't keep him.