Now that he is on proper formula with the correct amount of calcium I would wean down the calcium. It should not be necessary to supplement calcium when on the formula.
Malnutrition. Severe malnutrition. How old is he and how much does he weigh? I do around the clock feedings for these compromised, malnutrition-ed little ones... have received more than my fair share of these kids. There are at least 2 threads on TSB...
Which antibiotics and why? If they tend to bloat it is better to feed 1/2 the normal feeding amounts but twice as often.
I second CritterMom's recommendation to stick with FV 20/50 but add Ultra-boost (1 part ultraboost to 3 parts FV 20/50).