I interpret the 'drugs don't work' as he is resistant to sedatives. I know a couple horses like that. The most common sedative being ACE. It either doesn't work at all, or it causes the horse to be even more anxious and upset, as is the case with the abused Arabian I work with. Some horses for some reason are just resistant to the effects of sedatives, and you obviously don't want to overdose a horse on sedatives, so it makes it very tricky to make something work. I personally would feed him yourself, water him yourself, if possible, sit outside the stall where he can still see you, and just read a book, do some homework, or whatever you can find to do that puts you in his viewpoint, but that helps you completely ignore him. Just being around him for a while without anything happening to him will go a long way to helping him feel calmer around people. And definitely make sure that you have done all you can to find his possible owners. It may just be a case of he got loose, is scared cause he doesn't know where he is, then he got stuck in a mud puddle, then drugged, then dragged out, and then put in a strange place with strange people trying to do who knows what to him, and all he wants is one familiar face. That's what is going on with my horse right now. Moved her to my ranch, and she's been in the same place her entire 4 years of life, and she gets concerned and upset, but she settles down once she sees me or my bf, as we are the only familiar things in her world right now.