I am so sorry for your loss.
May Buddy rest in peace.
A necropsy would answer your question on the cause of his death. You may want to consider this option, considering you have other birds.
Please accept my condolences.
I am so sorry to hear about Buddy It must be so hard to lose him when you least expected it. Let us know if there's anything we can do to make things easier on you. Rest in Peace Buddy, knowing how loved you were and still are!