Did you force feed him the electrolyte water? If so did you use a tube? Some could have gotten in his lungs by accident. I wouldn't worry about that unless he is gasping a lot for air.
I think that the green poop and lack of strength could be coming from him not eating enough, which could be from the mite infestation or something else. You may also want to treat them with corid just incase they are overrun with cocci.
It can be a little difficult to tube feed a chicken for the first time. Just take it slow and have a helper. At the base of the tongue there is a slit that can open wide, you dont want to get food in there so you'll have to go past that. I stick it in and see if the chicken has any trouble...
I gave her some penicillin at the same time, but yours may not need it. You just mix the formula powder with some water and fill the crop up. You may want to mix it with some chicken electrolytes or Pedialyte too.
A lot of pet stores have it, ill send a link to what I use, it was extra from some parrots that I raised. https://www.petsmart.com/bird/food-and-treats/pet-bird-food/kaytee-exact-hand-feeding-baby-bird-formula-3819.html#:~:text=Known%20as%20the%20first%20and,and%20all%20other%20baby%20birds.