Harry put the plates of egg and bacon on the table, which was
difficult as there wasn’t much room. Dudley, meanwhile, was counting
his presents. His face fell.
“Thirty-six,” he said, looking up at his mother and father. “That’s two less than last year.”
— Harry Potter and the Sorcerer’s Stone by J.K. Rowling.
It seems like the highlighted part mean he disappointed. But I don’t find any account for it in dictionaries. What does it exactly mean?
You’ve more or less got it right. “His face fell” is a common idiom which means “His face changed to a look of disappointment”.
For dictionary definitions try Oxford:
[no object] (of someone’s face) show dismay or disappointment by
appearing to droop: her face fell as she thought about her life with
6 e : to assume a look of shame,
disappointment, or dejection his face fell