If you look on the elevenplusexams forum there is a brilliant thread every year that tracks this: what the pass mark was that year, and then people on the WL post their DC's mark, and gradually you watch it move as more and more people get places.
For Tiffin girls, I think in the past two years, something like 25-35 people got waitlist places - most quite early (eg within a month or so of national offers day) but some not until Sept.
If you have a look, I expect there'll be a similar thread for Tiffin boys. But it'll really comes down to what his score was, and what number he is on the waitlist.