I would give the Terramycin oxytetracycline orally at least at first, since it looks like he would not drink enough of the water. Tylosin is avaliable here, but would take several days for shipping:
https://www.jedds.com/shop/tylan-soluble-100-g/
For a respiratory disease, I would give tylosin, doxycycline, or oxytetracycline. The scours tablets are oxytetracycline and I believe they are 250 mg tablets. I would use 1/4 tablet crushed in food twice a day for 7 days. But if you could get tylosin or Tylan, that could be easier to use.
I...
Oh, he looks very sick and weak. Did your chicks come from a hatchery, breeder, or did you hatch them from your eggs? Where are you located? I would worry that he or she has a respiratory disease, but worms could also be a problem. Do you have any antibiotics or wormer?