If it was hard, but you suspect is was his neck, I would say it was his neck. If there was anything in there, it would feel like a ball of stuff. Maybe pick up one of your other chickens and check them. Or wait until they are on the roost and you can feel without even picking them up. They should be pretty full at roost time, so it will be pretty clear what it is. And yes, you can check him while he is crouched.
I'm not sure that Pen G will be effective for whatever he has but if it is some infection, it might. Just make sure that if you do start him on it, to give him a full round, regardless if he shows improvement or not.
Have you smelled his breath? That can be an indicator of some illnesses. Just trying to cover anything that may have been missed so far...
How long has he been like this now? Does it seem like it's getting any better or worse?
In the event that you are not able to save him, I would really consider getting a necropsy to determine what happened. I only say this now because you have a short window to preserve the body and get it off to a lab. If you do a search on here, you will find some good articles detailing how to prepare them for shipping and there's a few suggestions on where to send them. There's a lot of colleges that will do necropsies for free or a fairly small charge. It's worth it so you have an idea of how to treat it the next time you have to deal with something like this.
