It is fun ________ (work) for him.空格处填什么

to work / working 都可以吧.
你这也没语境 .
fun当名词时,
当“享乐,乐趣,快乐,享乐的事”的意思讲时,
有be fun doing
eg: It's not much fun going to a party on your own.
(独自一人参加聚会没什么意思）
fun当形容词时,
当“逗乐的,有趣的,使人开心的”的意思讲时,
有be fun to do
eg: She's really fun to be with.
(和她在一起真开心）