I feed my bird whole apples, seeds, & all.You said he was being fed apples? Would that be with the cores? Apple seeds contain cyanide. I'm not sure how many he'd need to eat to have an effect, but would he have had access to the seeds? With him being unsteady on his feet, I'd worry about him getting ahold of something toxic or having a vitamin deficiency, not to mention his age being when many of the nastier poultry diseases can manifest.
The cyanide actually acts like a dewormer, it's not concentrated enough to harm the birds, but can rid them of some worms.
Ducks are more sensitive to cyanide, & arsenic, which used to be an ingredient used in early formulas for treating Coccidiosis infection. This caused many ducks to die during treatment.