I'm sorry to hear that and know how you feel. That was a beauty of a cockerel and have lost a few favored roosters over the years. Amprolium is always good to keep on hand. The same with vitamin, probiotic supplements, and a few good antibiotics like Amoxicillin, Cephalexin, or Baytril...
Yes. use the dose recommended previously for the sick one. Don't use medicated feed. I would begin with a preventative dose of Corid 9.6% at 1.5 tsp per gallon for 5 days, every 3 weeks until they are all at least 7 months of age. After each run of Corid, use vitamins-electrolytes, and...
Straight dosage out of the bottle of Corid 9.6% is .2 cc for every 2.2 lbs of the bird's weight. A 1cc syringe without a needle will work well for oral feeding.
Young birds are at risk as their immunity develops. The best you could do is take the bird inside to a warm and comfortable environment. You will have to hand feed/ water the bird if it is not drinking/ eating on its own. That is where a syringe comes in handy. Moistened feed, scrambled egg are...