Jair is called here the son of Manasseh, partly because he was so by his mother, 1 Chronicles 2:21,22; and partly because he joined himself with the Manassites in this expedition, and settled himself among them.
Related Commentaries of Numbers 32:41
Numbers 32:41
41 And Jair the son of Manasseh went and took the small towns thereof, and called them Havothjair.