Welcome to Welsh research - so many names are the same!! it can be really confusing.
I had a look at the 1911 census for you and I have found a William Henry that could match. There are three but this is the only one whose father is John.
He is aged 12 living with his parents John (43), Coal Miner/Hewer and Mary (42) along with 8 brothers and sisters at 3 A Dare Terrace, Treorchy. His parents are both from Llanelly, have been married 19 years, and had 13 children (9 living and 4 dead by 1911). This could be him but then again you are trying to do research on the most common surname in Wales. Hope this helps.
Julie (Williams - 2nd most common Welsh surname)