In the first ruling of its kind, a judge said a vasectomy would be lawful and in the best interests of DE, who lives in the Midlands and lacks the capacity to make the decision for himself.
High Court judge Mrs Justice Eleanor King said DE's 10-year "remarkable and precious" relationship with his girlfriend had nearly broken under the strain of the couple being kept apart to avoid an unwanted second pregnancy following the birth of a child.
She ruled the overall picture pointed to re-establishing as normal a life as possible as soon as possible for DE, now nearly 37.
The judge said the birth of the child in 2010 had a profound effect on the families of the couple and led to a ban on thembeing left alone together.
The girlfriend's learning disabilities were not as severe as his, but she was unable to look after her baby by herself. She lived with her own mother, who had been declared the baby's special guardian.
DE struggled to understand what was going on and denied the baby was his.
But he had been seen to play with his son and call him "my child, my child", while making it clear he did not want another child.
For three to four months PQ ended their relationship "to his considerable distress" following a ruling he did not have the capacity to consent to sexual relations.
The relationship resumed in March this year, and by July parents and experts agreed that DE could consent to sexual relations, but could not consent to contraception.
The application to allow a vasectomy was made by DE's local NHS Trust.
Learning disability charity Mencap welcomed "the emphasis placed on the fact that this is an exceptional case and should not be seen as a green light for other applications for sterilisation in respect of people with a learning disability".