I would continue with the shots -- the Penicillin can't hurt him, and may help him.
If he is eating grass, but nothing else at least he's getting 'nature's horse food'. I would also suggest some form of ulcer treatment -- as it would coat his tummy and help ease any discomfort. Perhaps one of the vets would let you pick up some Gastroguard or something similar that you could give him. I know when any of my horses go off their 'feed' I automatically treat for ulcers and have had good results with the treatment within a few days. In his case, it's probably not ulcers, but perhaps his tummy would feel better. Even simple Maalox syringed into him could help.
Is Alfalfa hay available to you -- that you might be able to get a bale and see if he will eat that? It is a tummy soother, and also has the protein he could use. If not, you might try alfalfa cubes or pellets, and soak them to get some extra water into his system. Keeping him hydrated is important, so monitor his water intake and make sure he's drinking.
I can't believe that you are having such difficulty with the vets. I have great difficulty getting a vet here these days -- my equine vet moved away, and the other local 'boys' just enjoy working at the big cattle and quarter horse ranches (more money), but there is one who will eventually come after his other work is done.
If you have a trailer, perhaps you could take him to a neighboring town vet -- since yours are not responding. Or try calling another nearby town and explain the circumstances and perhaps some one will come.
I'm so sorry Tango is still having some difficulties. But if he's eating and drinking -- those are positive signs. Monitor him for any fever. The Penicillin should take care of any bacterial infection he might be having. I would keep up the supportive measures of water, grass and add the ulcer treatment. All may help him get through whatever this is.
JMHO
Lots of prayers coming to you and Tango!
Thank you for the advice.
So far Tango hasn't had a fever or shakes. He never got nearly as sick as Lemby. He's out eating grass right now and I've seen him drinking plenty of water. I just gave him some apple, which he loves and he ate those. He also took a couple of bites of grain. My husband is going to stop at a vets office and see if they'll give him some gastroguard. He just looks so lonely and sad out there by himself.