How can I effectively coach my 5th-grade son for his upcoming general knowledge test, which needs to learn animal names according to the number of letters, and animal names starting with specific letters?
Any help?

I remember the good old days when I was used to such surprise tests
First, ask him to go through any notes if he has otherwise found some online platforms, for example, if needs 5 letter animals to learn the names from this list: https://www.wordsdetail.com/5-letter-animals/
You can find 3 to 12 letter and A to Z animal names here.
Secondly, make it fun with some games, flashcards, or quizzes to keep him engaged and motivated.
Wish you the very best of Luck