Rooster Booster is a brand name. The product I would choose that they make would be the Poultry Cell.Thanks again. All the supplement I could find at Tractor was something called “rooster booster” seems like a multivitamin for chickens. I also have him running on the chick booster that has some vitamins in it as well.
Marek's vaccine is not 100% and depends heavily on initial quarantine.
New Castle's might be considered rare... but tell that to all the folks in California who were first under mandatory quarantine not able to get any chicks or birds this year... but worse it was followed by mandatory euthanasia of ALL back yard flock within part of the quarantine! I don't personally think that's your symptoms, but just like to make the point to not think "it can't happen to me".
Your vet assessment of the rooster not being malnourished... um, well he isn't deprived of energy (starving or skinny) but he *may* not be processing a specific nutrient appropriately. It's great that coccidiosis wasn't an issue that could have caused long term problems like this due to intestinal necrosis.
I see lots of birds living with torticollis for several years, quite contentedly it seems... locally there is a Pekin drake named Hunker Duck for how it looks like he's hunkered down.
Keep up your massages and helping him keep or gain back his movement if possible... as it will likely be beneficial to his recovery.