If you know he's reading a book, yes, as 看 is literally meant "read" in this situation - 看小說,看雜誌,看報紙,看宣傳單,看漫畫書, all are the applicable response but very specific about the material/item he is reading. Otherwise, you can say "他在看東西", which is similar to "他在阅读“, but not as formal.
However, there is a catch though, as "他在看東西" can be misconstrued as "He is watching something (from an electronic device)", or "He is (literally) stare/look at something" by the listener. So be prepared to provide further clarification, as the inquiry "他在看什麼東西?" is likely to follow right after your answer.