« Reply #6 on: Monday 19 September 22 17:54 BST (UK) »
Where and when were the other children born?
It may be possible to find them on the census with more information and so find their mother. Or find the birth registrations and therefore the mother's maiden name. There's more than one David John Rees birth registered Pontypridd of an age to be 1 on the 1901 census.
By the way, no boarder on the 1901 census, Joseph is a visitor and he's the grandfather of Rees.

Cornwall: Allen, Bevan, Bosisto, Carnpezzack, Donithorn, Huddy, James, Retallack, Russell, Vincent, Yeoman
Cards: Thomas (Llanbadarn Fawr)
Glam: Bowler, Cram, Galloway, James, Thomas, Watkins
Lincs: Coupland, Cram
Mon: Cram, Gwyn, John, Philpot, Smart, Watkins
Pembs: Edwards (St. Dogmael's)
Yorks: Airey, Bowler, Elliott, Hare, Hewitt, Kellett, Kemp, Stephenson, Tebb