As G55 says, he can stay on the waiting list for a year until the 12+ pass "expires". He would then need to take the 13+ in February and qualify in order to stay on the waiting list for a further year.
The 12+ result "expires" once the 13+ result is known and supercedes it. I have to say I am posting that last sentence purely because that is what common sense dictates, rather than because I have seen it written down anywhere! There are so few children involved in the process at that point that I have never put the rules under my usual microscope.
He can stay on the list regardless of where his current school is - Timbuktu is fine, but I am guessing he will be a little closer?
From what I hear in recent months, you may be quite happy with BGS, although the inconvenience will still be a major factor for you.