Onus is on the claimant to show that the highest NDD is not applicable. It may be helpful to suggest that contact details for the son are obtained and the latter could then detail what has or has not taken place but there;s no obligation to do so. Very unlikely that anyone in the sons position could get someone above them to confirm the when and where of active duty. Simply not the done thing within the forces for obvious reasons. Absent that then the highest NDD applies. The barest of information from the son would ordinarily suffice unless it contained obvious contradictions etc.
