Do you have a copy of the marriage of Joseph Harold FREDERICKS and Brydah RATTLE because that should give a first name for his father? I note that on FreeBMD the marriage was registered in Pontypridd in the June quarter of 1913 under the name Harold FREDERICKS.
You mentioned that his father had died during the Boer War - There are records relating to the Boer War available on both Ancestry and Findmypast but without a first name it is difficult to decide which is the correct record. However, there is a death registered in the GRO Natal & South African Forces Death (1899 to 1902) of a J. FREDERICKS of the 2nd Battalion of the East Yorkshire Regiment - He died in July 1900 at Springfontein from disease - Unfortunately I can find no service records for him.
I haven't found a birth registration for a Joseph Harold FREDERICKS - in fact there are no births registered with the surname FREDERICKS in Somerset at that time. There is a Joseph Harold FREDERICK whose birth was registered in Kensington in London in 1893. Do you know whether he had any brothers or sisters?
Have you considered whether Joseph Harold might have been illegitimate and therefore registered under his mother's name? This is a bit of a longshot but in Weston-super-Mare in 1911 was a 23 year old Dentist called Harry TOWNSEND who was born in North Petherton, Somerset.
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