The phrase "activities when he was" is correct and usable in written English.
It can be used to refer to specific actions or events that occurred during a certain time in someone's life.
Example: "He often reminisces about the activities when he was a child, playing outside with his friends."
Alternatives: "events during his time as" or "things he did when he was".
But she said Mr. Trofimoff was seeking payment for espionage activities when he was arrested.
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.
Mr. Farkas said that, while he could not discuss specifics of his client's efforts for prosecutors, he understood that Mr. Castro's activities, when he was recording, were generally focused on "specific missions".
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.
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.
10 P.M. (Sundance) BREAKFAST ON PLUTO (2005) In this film by Neil Jordan, a sweet Irish transvestite (Cillian Murphy) transplanted to 1970s London finds himself accused of terrorist activities when he is suspected of bombing a disco.
[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.
Leo's brain was shrinking very fast: his brain activity when he was wide-awake was equivalent of that of a normal person asleep.
The soldier, from Corby, was on patrol to disrupt insurgent activity when he was caught in the blast of a grenade.
