Best course is not to panic. Encourage your DS to keep trying to improve until the last exam. Forget about the grammars which are not in your catchment. Concentrate on the ones within reasonable travel.
Also do not undermine the comprehensive schools that your DS may get in case he doesn't get place in grammars. The Sutton grammars would conduct second stage tests by mid October, but St Olave's exam is bit later and who knows he may do well there or in other grammars.
Also remember that it's not end of the world if one does not get into grammars. Jeremy Corbyn hates his grammar and left with two E grades. He is Leader of A major political party.
Your DS could also try for independent schools. He may even get scholarship if he keeps improving till their exams in January. You may look for bursary. Whitgift and Trinity schools are in Croydon and few others.