What we do to prevent lice and mites in the coop in the first place is religiously sprinkle food grade diatomaceous earth (DE) about twice a month throughout the coop, nests, and dust baths.
I've never had a chicken get them, but if I did, I'd reach out to something stronger as DE is more a...
I was going to suggest a regiment of Poultry Cell and probiotics until I saw the part about the lice. I'd still do the vitamins, but the lice could be your culprit as they can cause anemia and other problems if not eliminated. I bet if you get him totally mite and lice free, that will help...