In the meantime, there are things you can do to help your son at home. You're right that sometimes people have difficulty remembering what they've read just because their attention is wandering. All of us have experienced that phenomenon: We read a page in a book or a story in the newspaper and become distracted by other thoughts, then suddenly realize that we don't remember anything we've read. The important thing is that we realize that we've lost the content. If we know it is happening, we can be ready with "fix-up" strategies to fill in the holes in our comprehension. 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. Franklin Electronic Publishers at 1-800-266-5626 has a host of user-friendly electronic dictionaries.
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. These "word pictures" make it easier for them to construct and then to retrieve meaning over time. 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. You're talking here about "procedural memory," i.e., remembering the steps you need to take to complete a task. Try creating a "math notebook" for your son where you write down the steps to solutions, with examples of completed problems. 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.
If you're not comfortable trying these strategies on your own, your son's teacher or the guidance counselor at his school may be able to recommend a tutor who can help him to develop more "active" learning habits. Good luck!