Louis, 24, posted a black and white photo of him holding his son, accompanied by the words: “Meet my little lad, Freddie.”
He hinted the child will have his surname, tweeting simply “Freddie Tommo” before following it up by saying: “Thank you so much for the all nice messages.”
The selfie shows the star holding the boy, wearing a striped babygrow, to his bare, tattooed chest.
Meanwhile, Briana plants a kiss on Freddie’s cheek from her hospital bed, describing him in the caption as the “love of my life”.
Soon after Freddie’s January 21 arrival, Louis also posted this message to his 7.8 million followers: “I’m pleased to say my baby son was born yesterday. He is healthy and pretty amazing. I’m very happy.”
Despite reports that he was banned from the delivery room by Briana’s mother, a spokesperson says he was in fact present for the birth.
“This story is absolutely not true and Louis was by Briana’s side the whole time in delivery room for the birth,” says the source.
Although they are no longer together, Louis and Briana, below, had a brief relationship in May, 2015, after Louis split from his girlfriend of four years, Eleanor Calder.
He is now dating Disney star and actress Danielle Campbell.
Louis is the first member of One Direction to become a dad. For an update on the group’s latest video for fans and news on OD’s hiatus, click here.