I agree with the other comments on retaking the SAT one more time given the selectivity of some of schools you are targeting. I also think the December 3rd test date will be too late to have an impact on Brown ED as decisions will already have been made before the school sees the scores.
There is a November 5th SAT that is still showing as open for late registration until October 25th on the College Board’s website, assuming you can find an open location that works for your son. I would suggest he takes that if he wants his score to be considered in the ED round.
Good luck!