Answer:
Hyperbole
The sentence means that Grayson extremely happy to have learnt how to read.
Step-by-step explanation:
Hyperbole refers to an exaggerated statement which is not meant to be taken literally. As a result, the sentence about Grayson is a metaphor, amplification or enlargement. The writer does not expect the reader to take the sentence truly; actually, he or she means that Grayson feels in such high spirits about learning how to read that he has a huge smile on his face.