The caption is a reference to the Netflix series The Last Kingdom, which Grimes joked about a week earlier when showing off the child's Medieval-esque haircut on her Instagram Stories.
"Not sure this haircut went well but he's Viking now," she wrote at the time. "Haircut inspired by the last kingdom on Netflix which is a masterpiece."
In another post on Instagram, Grimes shared a picture of herself fondly embracing X Æ A-XII, who looks on at what appears to be a statue of the futuristic Maschinenmensch robot from the 1927 film Metropolis.
"Tell me of the waters of your homeworld," Grimes captioned the image, quoting the sci-fiction novel Dune.
The indie pop star and the SpaceX and Tesla CEO welcomed their first child together in May 2020 after roughly two years of dating.
See Grimes with X Æ A-XII here and Musk alongside his son here.