Michael Biggs said his father, who has pneumonia, needed minor surgery and would not be fit to move until at least mid-August.
Justice Secretary Jack Straw said late on Thursday that he had decided to release Biggs on compassionate grounds.
Biggs, who will be 80 today, is being treated at the Norfolk and Norwich hospital in Norwich after being moved from Norwich Prison.
Speaking outside the hospital, Michael Biggs said he expected the Ministry of Justice to confirm his father’s release late last night.
“I am a psychological wreck at the moment,” said Michael Biggs. “I have never known my dad as a free man. I have only known my dad as someone who was either wanted or on some sort of parole.”
Michael Biggs said he expected his father to undergo minor surgery on Monday or Tuesday.
Biggs, from Lambeth, south London, was a member of a 15-strong gang which attacked the Glasgow to London mail train at Ledburn, Buckinghamshire, in 1963, and made off with £2.6 million (€3.05m).
He was given a 30-year sentence, but escaped after 15 months and spent 30 years on the run living in Australia and Brazil — where Michael Biggs was born.
Biggs returned to Britain voluntarily in 2001 in search of medical treatment and was returned to prison.