The couple is no longer together and reportedly learned about the baby after they had parted ways.
"They used to date but it wasn't serious," says a source.
Apparently, Jeremy was initially "being ultra-secretive about [the pregnancy], but [Sonni] has been going on about her life and not hiding it."
However, Jeremy's pals are quite happy for him.
"Jeremy will make a great dad!" says a friend of his.
Jeremy has had a busy year, as he starred in such films as The Avengers, The Bourne Legacy, and Hansel and Gretel: Witch Hunters. He previously had earned Oscar nominations for The Town and The Hurt Locker.
Congrats to Jeremy and Sonni! A baby is quite a legacy.