Hello Sharon,
What i would advise is that at this stage you need to contact Pontypridd register office and ask them if they can find a record for David John Williams born 17 March 1915 in or around Mountain Ash. If they cannot find a record then your next point of contact will be Merthyr register office. Obviously trying to find someone with the name of David Williams is very difficult, to give you an idea there were two births with the same name registered in the Pontypridd district in the first quarter of 1915, that is assuming they registered the birth before the end of March. The number for the office in Pontypridd is 01443 486869, or the email address is registrar@rhondda-cynon-taff.gov.uk
If you do find the record then you can cross check the match of the mothers maiden name against any records for his brothers and sisters to be certain.
Best Wishes,
Tallpaul.