Ian, how have you learned of the missing son, and is your informant/source clear on the dates and details?
The Tony whose dates you have given seems to have been born Tony Sidwell, to parents Stanley Sidwell and Lillian née Jennings, and become a “Brett” on Lillian’s subsequent marriage to Wilfred Brett in 1933. All of these events were in Lancashire. As Annette says, there’s no obvious link to your Bretts, unless you can shed any further light.
how have I learnt of the missing son?,
The Tony Brett I'm posting about is mentioned in my material grandmother's death notice as her brother and he died in 1987, how I came about the date 1928 I learnt from freebmd and gro.govt.uk and ancestry ..
there is no birth certificate on freebmd and gro for Tony..
this makes me believe his mother is my material grandmother as she would've been 21 at the time of his birth and had Tony out of wedlock before marrying a guy in July 1928