"在他闲暇的时候" 可以翻译为 "in his free time" 或 "when he has free time" 。例如：
In his free time, he likes to read books.（在他闲暇的时候，他喜欢读书。）
When he has free time, he often goes hiking with his friends.（在他闲暇的时候，他经常和朋友去徒步。）