Complex sentence: In his teens, he became an "uchideshi" (live-in student) of Morihei Ueshiba, the founder of aikido during World War II and also spent a brief period at the Aikikai Headquarters dojo after the war.
Split into simpler sentences: In his teens, he became an "uchideshi" (live-in student) of Morihei Ueshiba, the founder of aikido. His live-in apprenticeship under the founder took place during World War II, and he also spent a brief period at the Aikikai Headquarters dojo after the war.