We are going through the exact same thing! Our DS13 has taken several aptitude and intelligence tests. His Math reasoning scores on various aptitude tests are in the 99% but Math computation scores are lower. We brought him to a counselor who specializes in gifted children and right away she zeroed in on the fact that he is a visual-spatial learner. After reading for two days all the literature I could get my hands on about visual-spatial learners, now I understand why DS is failing Math. Gifted visual-spatial learners understand the concept but given the way their brain works and learns, it is next to impossible for them to show their work.
The problem with visual-spatial learners is that the regular classroom setting and instruction focuses on auditory-sequential learners; these learners are at the opposite end of the spectrum as visual-spatial. Simply, the regular classroom setting neglects visual-spatial learners and they fail to succeed.
Maybe reading up on visual-spatial to see if your son matches that learning style would shed some light on his Math issue...
