Ohhh no, poor little guy
I will try my best to help.
Has he got any weight loss? Impacted crop? Any signs of pasty butt? Parasites? Signs of injury?
What does his poop look like? Any blood? Has he had access to water at all times if it has been hot? Has there been a sudden change in temperatures? Has he got a rotten food smell coming from his mouth?
The mites could very well be a possibility, it can slowly kill a chicken. Search for mites, everywhere on him, on the neck on the back in the wings etc. And if you see mites my first step would be to get rid of them, all.
In fact, I have a roster right now who was showing signs like your boy is, he's been isolated in his own cage he had a HUGE mites infestation, so if your boy has mites, I will tell you how I treat them.
Get three buckets ready, doesn't have to be buckets.
In the first bucket/soaking area put----- Strong (not too strong) warm salty water, soak him in there for a good 5-7 minutes
In the next bucket/soaking area put-------- Apple cider vinegar/warm water soak him for another 5-7 minutes
In the last bucket/soaking area put-------Warm soapy water soak him for as long as you think, then rinse the soap off with warm water.
Then blow dry him. That's how I got rid of most of my rooster's infestation of mites. Repeat the next day if necessary.
Next up I would be putting some garlic pieces down his throat, not too big of a piece, and only about 2, garlic works wonders.
I would be making up electrotypes for him, and I would give it to him full strengh, don't mix it in his water. Here:
http://www.communitychickens.com/homemade-electrolyte-recipe-hot-weather-and-acidosis/
As for his water, put a tad of sugar/honey (Honey would be better) in it, and A.C.V it will help fight whatever is going on in his body.
As for feeding him, will he eat? I could list all the 'meals' to feed him but then the post would be too long, so here's a thread:
https://www.backyardchickens.com/threads/best-healing-food-for-sick-or-injured-chickens.813191/ So, in other words, i wouldn't be feeding him his normal chicken food.
Best of luck,
-Mustang