English Translation
He lost his only son in the war.
FYI: 亡くす is exclusively used for loss of a (beloved) person. You cannot use 亡くす for inanimate objects such as a key or a bag. You need to use 失くす (also pronounced as なくす) for such objects.
FYI 2: 忙しい（いそがしい） means “busy/hectic”. The Kanji 忙 consists of two parts: 忄（りっしんべん、立心偏; meaning “heart” and “mind”）on the left side and 亡くす on the right side. We regard the status of being busy as “to lose your sober mind”. Don’t you think that’s a creative way of thinking?