In warmer climates you should treat for Coccidiosis and parasites twice a year.
No one told me that and birds suddenly started getting sick and dying around 18 months old. I mean fast too. I had 5 die in a week. I went through 6 months of birds dying suddenly before I found out. I later found out none of the vaccines were 100% (more like 50%) effective. They help, but they aren't the great protectors we put our trust in.
Either one of these can effect his absorption of nutrients. Lack of nutrients causes neurological symptoms and death. They starve to death eating normally.
He needs Amprolium (Corid), a dewormer and multivitamins (especially E, B and D), afterwards (two days) he'll need probiotics.