It should be that He has a National Insurance Payment Record, that would appear on the HMRC Website. A sudden variation of National Insurance Payments, for a given year, might point to a change in employment.
If He is not Registered for the HMRC Website, then this would have to be undertaken, to view the N.I. Record.
Add: If Your Grandfather is in receipt of a RAF and/or Osram Occupational Pension, then contacting the Pension Administrators, should be able to provide the information, You are seeking.
If all of the above suggestions are not viable, then as 'nanny jan' suggests, applying for His RAF Service Record maybe the only relevant course of action.