You will not find any service records online for anyone who served with British Forces after 1923, those records are still with the MoD and you will need his service branch and ideally his service number; so get the family digging around for old letters or WWII items like pay book or discharge certificate.

For Frank Snr do you know what branch [Royal Navy, Army] he served with?

Do you have his date, place of birth and the area he was living when he joined up?