I think you should respectfully ask the panel if they would consider your son's ability and academic potential rather than his actual achievement to date. You could gently point out that the 11+ is probably meant to be a test of ability and potential.
Secondly, you should draw the panel's attention to the fact that your son is stronger on the maths side, and the 11+ (limited as it is to VR) may not have given the most rounded picture of his ability. Be careful, though, not to go so far as to criticise the system!
DS also has had numerous reports from school bemoaning his lack of attention and care, however he always seems to know more than DD
No harm in briefly mentioning the twin. Although not strictly relevant, it might get you a bit of sympathy .....
And the use of words such as "numerous reports from school bemoaning his lack of attention and care" will tell the panel that you're being completely honest with them!