Just found this guy?

[Gotalotofpetstoo]

Looks like a Barred Rock cockerel to me.
Spurs look relatively normal, not cut.

I'd feed him the Chick Starter which should be perfectly fine for him, offer it as a wet mash if he's not eating very well.
I'd only introduce the feed, nothing else, while he may be used to free ranging and eating a variety of things, I would stick with just plain 'ol good chick starter, nothing more.
You want him eating on his own. Electrolytes or sugar water may entice him to drink.

Look him over for lice/mites. If possible get a fecal float to see if he has worms.

The bottom of the feet don't look too bad and are pretty normal as far as coloring goes since they've been cleaned up.

Comb may have a slight bit of frostbite, hard to know, could just be he's not feeling well, has been picked on...time will tell how that turns out.

I do see some crusty material around the eye. Take note if you see bubbles or watery eye, mucous from the nostrils, any coughing, sneezing, etc.
Definitely keep him away from your other birds for an observation period.

Another thing I just noticed, when he drinks, he is not putting his neck up to swallow. Is this unusual?
https://photos.app.goo.gl/A14hjoiUUnEMGLEu8

 

[Gotalotofpetstoo]

Thank you for rescuing him! Unfortunately, if he was abandoned, the rest of his flock will likely end up in the same predicament as him over time (or get eaten by predators).

The others ran when my husband approached, so it might be a challenge to catch them, but we are definitely going to be keeping an eye on them.

 

[RoostersAreAwesome]

Another thing I just noticed, when he drinks, he is not putting his neck up to swallow. Is this unusual?
https://photos.app.goo.gl/A14hjoiUUnEMGLEu8

Looks like he wants to drink, but isn’t actually swallowing most of it

 

[Wyorp Rock]

Is there a bad odor coming from his beak?

I see yellowish lesions/bumps and some mucous. Could be respiratory infection, perhaps Canker.

 

[Gotalotofpetstoo]

Is there a bad odor coming from his beak?

I see yellowish lesions/bumps and some mucous. Could be respiratory infection, perhaps Canker.

We had just fed him so he still had some residue of food in his mouth. But I'm sad to say he passed away this morning.

 

[Wyorp Rock]

We had just fed him so he still had some residue of food in his mouth. But I'm sad to say he passed away this morning.

Oh dear, I'm sorry to hear he didn't make it. How sad

 

[5OneChick]

Omgosh just read your story and found it inspiring.
Bless you and your husband for taking Duke in and trying to save his life. Seems like you received some really great support here.
I still feel newish here on BYC probably bc I learn so much every time I log on and so happy to be part of this community.

So sorry to read that he passed away despite all your care and attention.