Domino Kirke shared a close-up shot of herself breast-feeding her and Penn Badgley’s baby boy on Monday, September 28.“#nationalsonsday,” the singer, 36, captioned the Instagram Story photo showing the infant’s hand on her chest.The English star also honored her 11-year-old son, Cassius, whom she welcomed in 2009 with Morgan O’Kane. Kirke posted a photo of the preteen on a hike.She announced the arrival of her and Badgley’s first child together via Instagram earlier this month.