Hi Michael Hill,
The guide to child benefit on our website does advise that if you choose to opt out of getting Child Benefit payments because of your level of income, you should still fill in the Child Benefit claim form and state on the form that you do not want to get payments.
This will ensure that any NI credits that are due are still awarded (
High Income Child Benefit Charge).
In your circumstances it is not possible to claim the credits retrospectively.
However, whilst the payment of Child Benefit can continue up to and in some cases beyond the child’s 16th birthday, the NI credits are only awarded up until the child’s 12th birthday (
National Insurance credits)
If your son left full time education in 2015, I suspect, even if Child Benefit had been claimed, no credits would have been due for the years you have queried.
Thank you.