6 is very, very young - his impulsivity may have diminished a bit by 8 - maybe enough to get him to actually try on the test.
I feel your pain because grade skipping and after schooling in Maths made our DD go from being a person as explosive as nitroglycerin with just as little provocation to someone far calmer (most of the time).
Try some after schooling in areas of interest and participation in physical activity or martial arts to give him an outlet and help his self discipline would be my wholly unqualified and non-professional advice.
