Is this new or has your son always had trouble with reading and comprehension? I think that once you know how to read well, the more you read, the better you get at it. My husband is dyslexic and when he was in grade school the remediation they used included a speed reading machine. His comprehension suffered and he came away feeling stupid because they didn’t address the cause of his slow reading. For struggling readers, it’s important to make sure there aren’t any underlying issues first, then work with materials they’re interested in to build their skills.
After our daughter was diagnosed I spent hours reading to (and for) her. We read anything that caught her interest. She’s now a teen and devours books faster than I can supply them. But, like her dad, the materials she reads fastest and retains the most information about are those she’s interested in. I can plow through material even if I don’t like it; but it’s very difficult for dyslexics to do that. If your son has trouble with comprehension, I’d address that first and worry about the speed later.