Mr Keyworth said Sandra was determined to give Charlie one last special present -- so she and family arranged for a star to be named after him.
The star, visible from New Zealand, has been dubbed The Badger, one of Sandra's many pet names for her only child.
"I'm not sure where the Badger came from; it might be an English thing, but it stuck," Mr Keyworth said.
"This is a neat thing for Charlie to have -- something tangible he can see and remember his mum by.
"When he arrives, we'll look out for it."
In June last year, Sandra travelled to the UK to receive Selective Internal Radiation Therapy (SIRT), which inserts tiny radioactive spheres into liver tumours.
She was given chemotherapy to shrink the tumours, to allow doctors to remove the cancer -- with SIRT considered a last resort.
In January, she was told the cancer was in 70 per cent of her liver, making it impossible for SIRT to succeed.
"They told her to go home and get ready," Mr Keyworth said.
"She said it was too late for regrets -- she wanted to enjoy the time she had left."
Mr Keyworth travelled to Birmingham to visit Charlie for his mid-term holidays, and was able to spend time with Sandra.
Though the couple had separated not long before her diagnosis, they had "a good catch up".
Mr Keyworth said Charlie is doing well, thanks to the support of his extended family.
His spirits were buoyed by a trip to Leicester's King Power Stadium not long after Sandra's death, where members of the Leicester City Football Club -- which his great-uncle Ken Keyworth played for in his youth -- gave him tour of the venue.
This was arranged by Mr Keyworth's brother Rob, who contacted the team on Twitter.
"He got to run through the players' tunnels holding a ball and stand in the communal bath," Mr Keyworth said. "His face just lit up -- he was buzzing." Mr Keyworth said he is focussed on being "the best Dad possible" for his boy.
"I've just bought a whole lot of cookbooks. I'm not terrible in the kitchen, but having them there for moral support will help."