What was the name of Tolkien’s first son, who became an editor of his writings?

Tolkien’s first son, who became an editor of his writings, was Christopher Tolkien.

Christopher Tolkien was born in 1924 and became the principal editor of much of his father’s unpublished and unfinished material. After J.R.R. Tolkien’s death in 1973, Christopher organized, edited, and sometimes reconstructed texts from the extensive papers left behind.

His editorial work led to publications including The Silmarillion in 1977, Unfinished Tales in 1980, and the multi-volume History of Middle-earth. These books made a large amount of Tolkien’s mythology and drafting history available to readers, although they were not all prepared for publication by Tolkien himself.

Christopher Tolkien is therefore distinct from Tolkien’s other sons, John and Michael, and from his daughter Priscilla. He also created maps and illustrations for some Tolkien-related publications. His role was editorial and scholarly: he preserved and presented his father’s papers rather than writing new Middle-earth novels as an independent author.