Thank you all for responding!
He is not a recent addition. He will be a year old April 17th. We have had him since he was a chick. Up til now he has been completely healthy! He did eat a little, but I haven't seen him drink. I will feed him some scrambled eggs and ACV water to see if that will help him. I haven't see his manure. I was gone for a little while from home and just got back. He seems to be a little perkier, though!
Marek's could be brought to your property by wild birds at any given time. It could even have been there already and not reared it's ugly head until someone's immunity was weakened by something else. On top of that my state poultry vet at UC Davis says "as far as they are concerned Marek's is in EVERY poultry environment".
Notice anything in his pupil shape or eye color? Any toes curling? He is not yet a rooster, as that term is saved for boys over one year old, just for informational purposes. Until then he is considered a cockerel.
Remember, in order for acv to work for its recommended purpose it must be "raw, unfiltered, with Mother". And be sure not to do too much or get your mix right, I don't know the standard mix rate. Drinking is most important. Dehydration is deadly. Sometimes not seeing it doesn't mean they aren't though...
Marek's isn't a death sentence per say. Some birds do recover. But it should be considered a possibility at this point and where I currently point my finger to.
Did you check him over for any injuries? If you hadn't found him when you did, it could have been a much uglier seen as many birds will take advantage in order to elevate their own place in the pecking order. And many times others will join in and it could even result in cannibalism. In this instance I do think separation was the right thing to do.
Supplementing vitamins MIGHT help if it's a vitamin deficiency. Either any (liquid) bird vitamin from the pet department, nutri drench, or Poly vi Sol baby vitamins with NO iron can be added. On top of that if it is vitamin deficit, selenium has been shown to be a huge help!
What is his NORMAL feed including treats and supplements?
He may have been under the weather longer than you realize. As prey animals they will hide anything as long as they possibly can in order to not become an easy target.