It could also be something like a type of defect he was born with, something we can't see, that develops symptoms later. I had a wonderful Cockatiel that a neighbor had given me because the poor girl started having seizures at 3 yrs of age, not often, maybe once every few months. My Tiels usually live well into their twenties so when she passed at 7 years of age, after a nice life of choosing a mate & raising some babies I helped her feed, she seemed happy for the 4 years I had her. One beautiful sunny day, after enjoying flight time to the salad bar, a perch island I always set up Sundays full of greens & shredded carrots. She flew back to the top of her cage, climbed inside and started shaking. Poor baby died as I held her, as I'd done so many times before. I did get the Necropsy and found that she had been born with an enlarged heart. Sometimes we never know what is happening or why, but the best we can do is give them love, care & a wonderful life while they are here, enriching our lives.