He has joined the Harriers now, so - fingers crossed - he can get a few competitions in over the summer.
He will need to make hay while he can. Here is for why:
Track and Field based on Rules 7 and 107)As his birthday is September 30th, this means that this is his last year at U13 whereas most of the other people in his class at school for example would have another season before going up to U15, and next year he could be running against a kid just 30 days short of three years older than him.
"The operative date for determining membership of age groups for all athletes under age of 17 shall be... the 31st August at the end of the competition year"
"The competition year ... shall extend from 1st October to 30th September for the following year"
Those who are aged 11 or 12 on the 31st August that competition year
Those who are aged 13 or 14 on the 31st August that competition year
Then again, them's the breaks. A line has to be drawn somewhere.