I have not seen that type of lesion on the skin either. You are treating as I probably would. I have seen dosage of amoxicillin as 57mg per pound given twice a day orally for 10 days. I would be thinking of giving Corid just in case of coccidiosis, but if your vet could do a fecal float plus a gram stain for bacteria, that would be best. I hope the chick starts to perk up with the antibiotic.