The Ivermectin killed the adults, right? You said nothing was crawling on him when you went to check last night.
So, adult mites died, chicken died, mite eggs hatched, baby mites are now looking for something alive to feed on. Hope you treated your nearby chicks.
To kill these mites, if they are still on the dead bird, poultry dust or whatever you have yo kill them outright. Raid? You still need yo treat all your chickens, because that rooster was surely not the only one with mites. They will feed on treated birds and die. Then you need to retreat in 7 days to kill off any new ones that hatch.