The important thing is that we realize that we've lost the content. This can be as simple as re-reading the text or clarifying difficult vocabulary. I've often recommended the use of an electronic dictionary to retrieve the meaning of challenging words. It's much faster than using a regular dictionary. People who have difficulty understanding what they re reading may use other strategies, such as stopping at shorter sections to make sure they are building meaning before they get too far lost.
Some people create visual images that transform the words they are reading into pictures.
Try talking with your son about strategies you use when reading challenging text to remember and understand what you've read. Talk through the process to make it explicit to him and get him to try some of these strategies in his own reading. The problem you're describing in math is somewhat different. Before he begins his math homework, have him look at the completed example and review in his own words the steps he took to solve it. My first grader brought home a note from school requesting that he participate in the speech therapy program.