The infant was just 24 days old when the dad first shook his son as he would not stop crying.
Less than a month later he did it again – and the child was rushed to Queen Alexandra Hospital as family members noticed a rash and the youngster vomited.
Medics feared the boy had meningitis – all because his father, who cannot be named, did not own up to what happened, said Kerry Maylin, prosecuting.
Portsmouth Crown Court heard it was only when the child was urgently transferred to Southampton General Hospital that medics discovered what happened.
Both the dad – who was ultimately jailed for two years – and mother were arrested by police.
The boy suffered subdural haemorrhages – bleeding between the brain surface and outer layer, and the fluid had to be drained.
When examined the boy was found to be suffering from sunsetting – when the eyes do not look straight ahead. Any long-term brain damage will only be found when he reaches school age.
The youngster has already missed developmental milestones and was prevented from seeing his mother when she was arrested.
She was later released with no action against her.
Reading the mother’s impact statement, Ms Maylin said: ‘At this point I didn’t think my life could get any worse. I had my baby son in hospital.’
And she told of the pain in being told by the dad what he had done. She added: ‘My son could have died because of what his dad did to him.
‘He is the one person who I should have been able to trust with our son.’
She added: ‘I hated him for what he did.’
Ms Maylin said the father, 22, had told the mother: ‘I did it, I’m really sorry. I never meant to hurt the boy. I love you, I never meant to hurt you all, I’m sorry.’
Andy Houston, for the father, said the child cried a lot and was difficult to feed.
He said the father was ‘immature’, adding: ‘Perhaps he didn’t realise the damage he had done in that first incident.’
Judge Roger Hetherington, sentencing, jailed the father for two years.
The father pleaded guilty to two charges of cruelty to a person under 16 years at an earlier court hearing.