"What is he doing?" is a common Engpsh question asking about someone's current activity.

Source: The question "What is he doing?" is a basic Engpsh inquiry that has been in use for a long time.

Examples:

"What is he doing?"
Translation: 他在干什么？

"Can you tell me what he's doing?"
Translation: 你能告诉我他在干什么吗？

"I wonder what he's up to."
Translation: 我想知道他在干什么。