The Blast has obtained a copy of the baby's birth certificate, which reveals that his full name is Ennis Howard Plemons.
Ennis has two meanings, depending on its origin: the Irish version means island, Harper's Bazaar UK reports, and the Gaelic means 'the only choice.'
According to Nameberry, it can be used for boys and girls.
The couple confirmed the news to the New York Post, with a source saying, “everyone is doing great”.
The couple became engaged in January 2017, and speaking to marie Claire UK in July she said she was ready for the next stage of her life.
“I’m at a stage in my life where I’m like, I’ve been working since I was 3. It’s time to have babies and chill,” the Beguiled star said.
“I wasn’t one of those ‘I need a baby!’ people until my goddaughter was born,” she said. “I love her so much. That love is just like … you can’t experience that unless you have a kid. I put her to bed last night and she woke up this morning and said to her mom, ‘Where’s Kiki?’ I just love that. That’s what I want.”
Kirsten announced her pregnancy with her dreamy Rodarte campaign, pictured above.