At the time there would have been alot of papaerwork created for the adoption, probably your aunt;'s mother had copies, are these papers still in the family? If you have an idea on the name the baby was registered in, as he would have had to be registered within 6 weeks using his birth mothers name, you can ask for a copy of his birth certificate from brixton register office, you then have the start of a clue, as the child will have been able to access their records from aged 18 and may know their history and not want to get in touch .