His pension says the 19th Kansas Cavalry. The 19th was formed to support General Philip Sheridan's winter campaign against the Southern Plains Indians. They were active from late fall of 1868 to the following spring. They were led by Kansas governor Samuel Crawford, who resigned his position to lead the troops. After forming, they moved south to Camp Supply in Oklahoma. After a harrowing trip, they arrived after schedule and did not join Lt. Col. George Custer and the 7th Cavalry in the Battle of the Washita. They did view the battlefield after the fact and helped with enforcing the peace. By March 1869 the tribes had surrendered so the troops were no longer needed. The troops moved to Fort Hays, Kansas, and they disbanded the next month.
The category you could use is Battle of Washita River (68) though they did not fight in the battle.
A coverage of the 19th's history can be found in True West Magazine: https://truewestmagazine.com/custers-other-regiment/
Kansas State Historical Society has digitized a handwritten history of the 19th http://www.kansasmemory.org/item/210758
