First of all heartiest congratulations to you and your DS for scoring such score.
I can feel your position with such high score.
Till 2014 entry, the catchment was enforced by 31st Jan of the entry. Reading School changed it to time of registration for current Year 7 (who appeared in Sep 2014 exam). If you were 2014 entry then you could have easily moved to catchment before next Jan.
it seems there are 189 students with passing scores inside the catchment. So if around 50 plus inside catchment pupil goes to some other schools then they will take from outside catchment and if there are only handful of interested students scoring higher than your DS, your DS will get a place.
Unfortunately no 13+ exams for RS anymore
but yes, if there are any dropouts in Year 7 then they call first 30 waiting list students in the catchment to appear for exam I heard.
So by moving into the catchment you might be able to get a place like that but there is no guarantee that there will be any place available after Year 7 in 2017.
One of our friend's son got into Reading School in 2014 Sep in Year 8 like that.