He went on the mission in 1966, before his stepson was killed in 1969.
From Wikipedia:
In 1966, Brigadier General James Stewart flew as a non duty observer in a B-52 on a bombing mission during the Vietnam conflict. At the time of his B-52 flight, he refused the release of any publicity regarding his participation as he did not want it treated as a stunt, but as part of his job as an officer in the Air Force Reserve. After 27 years of service, Stewart retired from the Air Force on 31 May 1968.[34]
Ronald McLean was killed in action on 8 June 1969, at the age of 24, while serving as a Marine Corps Lieutenant in Vietnam.[47][48]
Thanks for the correction.