Cute 'lil Guy!
Wonder if he is constipated. I'd work on hydration, if he's not drinking on his own, then tube or syringing fluids into him will help. Offer wet soupy feed.
I understand about having different ages within a flock, I do too.
I feed an All Flock feed which can be given to just hatched chicks all the way through life. I use anywhere from 18-22%protein feed. Oyster shell and grit (crushed granite) are provided free choice.
For the 'lil Guy, I would give him just the grower feed for now and omit the layer. While the excess calcium of layer feed may not be what's causing some of his symptoms, it's best not to give growing chicks too much calcium, Chick Starter, All Flock, Grower, etc. all still have some calcium, just not as much as Layer formulas.
Are all the lice/mites gone from him?