olster, for credit towards the UK NPPL with SSEA Class Rating, see CAP 804 Part II section 5 part A Appendix 1 pp5-6 (.pdf 863&864 / 930).
The UAS should really know this as the credit has been in place for many years now - but please note that only PIC time or flight instruction conducted by QFIs or civil FI(A) holders will be credited - air experience time will not.
Regrettably, thanks to the loss of the previously excellent military accreditation system, there is currently no credit towards either the LAPL or PPL. Although once your lad has his NPPL/SSEA, he can convert it to a LAPL (or PPL) in the same manner as any other NPPL holder.
If he hasn't already done so, I recommend that your son buys a civil logbook and enters all qualifying flight time. Note that he should use the 'chock-to-chock' time recorded in the flight authorisation sheets to calculate flight time in a civil logbook, rather than just the RAF's take-off to landing time. The UASs do so little flying these days that it shouldn't be difficult to find the old auth sheets...