After thinking about it for a couple minutes here, I think he ate something toxic to him. There may be no saving him at this point. It is so hard to diagnose over the computer with just words going back and forth.
Eating something toxic would cause him to go limp, legs, wings, and neck. He would not be able to swallow well and mucous would gather in his beak and he would in essance, drool.
The only thing that could be done in a case of eating something toxic is to give fresh water, do a molassis flush or an epsom salt flush. These flushes would require you to use a syringe and fill his crop with the mixture and then just wait and hope he passes whatever is ailing him.
Again, this is only my best guess at this point, I am not a doctor nor can one properly diagnose over the computer. If there is anything else you can share about his condition, please do so.