This happens in kids at the age of 5-6, when they have 10 thousand things to say, but only 1 mouth to do so. They start filtering the information as to what to say and it looks like a delay in response to us. This is very common and happens to most of the kids.
kids also become very slow in responding under helicopter parenting. There are kids who don't like to share a lot and resist the whole idea of having to tell what they play, what they do in school.
However, we shouldn't rule out learning skills issue without assessing him properly (memory, sentence formation skills etc), as you are saying he finds it difficult to perform in academics too. Can you come for a consultation with him?