It's a great concept, but teachers must give these kids the extra help so they can learn to proofread and spell. Because they have not been exposed to the correct spelling, many children don't look at a word and say to themselves: "Hey, that doesn't look right." Because your son is apparently a good reader, he is more likely to see the correct spelling of a word than another child who is not exposed to the correctly spelled word.
Since he still doesn't catch his errors, it could be due to two factors. He may not think that this is very important, since his teachers never inspired him in the proofing stage (which was supposed to happen eventually with inventive spelling, and which is often a part of what's called a "whole language" approach to language arts). These kids often inaccurately report: "My teacher doesn't care, so why should I?" Or, your son could have a type of learning disability, which is characterized by poor visual memory, or difficulty taking a mental picture of the sequence of letters in a word. If this is the case, he may spell words phonetically (the way they sound), and not the way they look. If he is a good reader, he is probably doing a very good job sounding out the words. Teachers can help students with poor visual memory or who have not learned the correct spelling by providing them with correct models, having them proofread their own (and others' work), and encouraging children to use spell-check features that force them to make choices.