If you get marked down in a recheck, can an already accepted college place offer be revoked if you now fall below the points threshold?

Yes the college has this right. In my opinion it would be highly unlikely that they would choose this course of action.

My son failed ordinary maths. If he requests paper to be checked and new examiner finds the should be lower, which grade stands?

The recheck result will be his new grade. However, if he is already taking a place up on a course and in October (post-recheck) experiences a five-point drop and now finds himself in a situation where he no longer meets the points required for his existing course, it will be the decision of the college that he is attending as to whether he can stay on the course or has to leave.

In my opinion it would be very unlikely.