I think that as long as there are no worries about appropriateness of subject matter, I'd let your son read whatever he wants. Not presenting material above a measured readiness level can help with not forcing a child to be frustrated with too-hard material, but that shouldn't apply to a kid who wants to challenge himself. It's a sign of a kid with a health drive to learn, and I think that the earlier a kid is exposed to complicated syntax and grammar, as well as advanced topics that are graspable with a bit of work, the better.
