The second vet diagnosed him with a vitamin deficiency. The cause being, wrong food. Where we got him from, unfortunately not a breeder, fed him cockatiel exact rainbow pellets since he was a baby and was able to eat it. So that's what we've kept him on. The vet gave him a supplement shot and also a form of supplement to administer to him. We're going to switch him slowly to some conure food. The vet said if he's not better within a week, to bring him back. I've been told Dante's slowly acting like his old self. Thank you everyone for your kind thoughts and prayers.

Edit: Zeus is perfectly fine.