Turkey, Pheasant or Game Bird starter that's medicated with Amprolium is what's recommended for keets. Other types of medications that are in starter feeds can kill your keets, so if you can't find one with Amprolium then I'd opt for something un-medicated. Chick starter does not have enough protein in it to help the keets to grow and feather fast... in my experience keets do a lot better on 27% or higher.
It would not hurt them to mix a little plain unsweetened yogurt in with some of their crumbles for a few days )or give them some probiotics) to repopulate their guts with good bacteria that the antibiotics most likely wiped out. I'd also dip his beak in the water w/vitamins on and off throughout the day for a while so you know he's getting a good dose of the vitamins, as well as plenty to drink.
Equilibrium issues can be from a nutrient deficiency, but can also be a symptom of a hard hatch or worst case scenario a genetic defect... hard to say what you are dealing with but just make sure he's eating, drinking and that he's plenty warm and hope for the best.