This depends on the child's personality and the level of difficulty of the books he's reading. It may well be that he's excited (or bored) and wants to get to the good parts, or that the book can really be read and understood while skipping some of the fine details.
Although you don't read that way, I would be willing to bet that if asked exactly what happened a few pages before (What kind of car did the woman get into? What color was her dress?), those relatively meaningless details would be fuzzy for you, too. There's a lot of fluff in most books. That's why the better books are better.
Some possible suggestions:
Get him books above his reading level, good ones. Newbery medal winners, or Caldecott medal winners. Read a few of them yourself and after he reads them, have a discussion (try to make it fun; over ice cream or something) with him. See how much he missed compared to you. If he didn't miss much, you can relax.
If you are still worried, there are books out there that test for all aspects of reading comprehension: meaning, context, implications, etc. on his grade school level. Invest in a few of these and do exercises daily (they don't take long) to see if you can identify any particular weaknesses he has (e.g. does he read too fast to understand implications?). The books should instruct you on how to work with your son on these critical (meaning evaluative, not important) thinking skills.
And yes, it's summer. He's giving a little bit of something up. Make it worthwhile (he gets to pick a family outing or something once in a while.)
Finally, you can just leave it to his teachers, who actually test for stuff like this, even early on. Talk to them about your concerns and see what their thoughts are.
It depends if you want to be "hands off", "hands on", or "very hands on". Either way, one of the most oft-cited statistics in how well a child does in school is how involved the parents are in their education.