Hello, and welcome!
The difficulty you face is this, from the Langley School Admissions Policy:
b) Eligible applicants with a permanent home address [See notes (c) and (d)] within the
school’s General Admission Area who are in receipt of free school meals at the closing
date for submission of the Common Application Form [See note (f)]
Your DS was not in receipt of FSM (even thought he was entitled to them) at the date you submitted the CAF.
Unfortunately I doubt that your application will therefore be reconsidered under that rule, even if you can provide proof of FSM going forward, and also his previous entitlement to them.
I am sorry to say that your only course of action from here is likely to be an Appeal. The outcome of that will depend on your overall case, along with other matters such as FSM.