What is he reading?
How long is the reading?
Is the reading in his level?
Now, often times, developmentally, reading and comprehension, is NOT in tandem occurring at the same time simultaneously.
Then their is fluency. Which means the ability to read fluidly and quickly without pausing while at the same time understanding what is read, all at the same time.
And this typically occurs, in a more instinctual manner, from about 3rd grade.
Your son is 7. 2nd grade?
Anyway, I would have him read, short things. A paragraph.
Then ask him about it. Or have him fill in the blanks of a worksheet, which is about the short passage he just read.
Versus, having him read a whole 64 page book, then asking him about it.
And when asking him about the book, be specific. ie: What was the boy's name? What did he do? Where did he go? etc.
Not just asking him "what was the book about?"
Then, there is comprehension per if they read something themselves. Or if it was read to, them. Or if it was read out loud to others.
And there is also, sequencing of events and if they can, chronologically say what the passage was about. What happened 1st? What happened next? type of thing.
At 7, a kid is also still trying to be grammatically correct/learn spelling/read/construct sentences/how to use adjectives/verbs/and still learning all those things. So per comprehension... it often occurs in STAGES. Not all at the same time as they learn sentence structure or spelling etc.
And, when a kid reads well. Good.
BUT keep in mind, that with young kids.... they are at the stage of READING and concentrating on reading. So then, "comprehension" is not the forefront, of their concentration. ie: just like when a baby is learning how to walk. They can't run yet. They are only learning how to walk and pace their steps and how to balance. They are NOT yet, "coordinated" enough to do and learn how to fully RUN, yet. It occurs in stages.
Just like with reading. Then, learning to read, then fluency and then comprehension... will occur in stages, and THEN it will occur in tandem.
And then in a more sophisticated manner.
But at this age/stage, they are still getting the knack of it.
I work at my kids' school and Sub as well.