The 27-year-old allegedly urged his teenage daughter to have an abortion after she posted a picture of her baby scan onto Facebook.
The youngster is thought to be five months pregnant.
Identities of the father and daughter are being kept hidden for legal reasons, but The Sun newspaper found that the child has been removed from mainstream education and placed in a special school set up for young pregnant women.
Her father was jailed when she was just two for knifing a bystander who was trying to prevent a robbery.
The previous youngest grandfather was Shem Davies, 29, from Brigend, whose daughter had a son when she was just 14.
He said in 2011: 'I'm about to be a grandad. I was 14 when my daughter was born, and now she's having a baby.'
'She is very young, but she was determined to keep the child.'
He added: 'We are not going to force her into doing anything else. We don't want her to hate us.'