The phrase "activity when he was" is not complete but can be correct and usable in written English depending on the context.
It can be used when referring to a specific activity that occurred during a certain time in someone's life.
Example: "He often reminisces about the activity when he was a child, playing in the park with his friends."
Alternatives: "event during his time as" or "task while he was".
[C2.] An Enlightened Moment at 9 Robert A. Watson, retired national commander of the Salvation Army, recalled attending a Salvation Army activity when he was 9.
L/Cpl Ashworth was on patrol to disrupt insurgent activity when he was caught in the blast from a grenade.
The soldier, from Corby, was on patrol to disrupt insurgent activity when he was caught in the blast of a grenade.
Leo's brain was shrinking very fast: his brain activity when he was wide-awake was equivalent of that of a normal person asleep.
Washington was unarmed and not involved in criminal activity when he was shot and killed in March 2010 in Koreatown.
By Ved Mehta The New Yorker, November 15 , 1982P. 51 PERSONAL HISTORY in which the writer, an Indian who lost his sight when he was almost 4, recalls his family and activities when he was 8 and 9 years old.
The New Yorker, November 15 , 1982P. 51 PERSONAL HISTORY in which the writer, an Indian who lost his sight when he was almost 4, recalls his family and activities when he was 8 and 9 years old.
By Ved Mehta The New Yorker, November 22 , 1982P. 123 PERSONAL HISTORY in which the writer, an Indian, who lost his sight when he was almost 4, recalls his family and his activities when he was 9 and 10 years old.
A senior government official who declined to be named said the investigation that produced Mr. Talansky's name was one looking into Mr. Olmert's activities when he was minister of industry and trade.
PERSONAL HISTORY in which the writer, an Indian, who lost his sight when he was almost 4, recalls his family and his activities when he was 9 and 10 years old.
