为什么一定是in both hands 而不是in his both hands

如果一定要加 his

用 in both of his hands

加上也对的！！！不过最好把both放在his前面，in both his hands.
不过在大家都知道怎么回事的时候 就不需要加了，避免重复 累赘。

如果加his要用in both his hands;或in both of his hands 前者不怎么常用
但不能是in his both hands
一般物主代词要紧跟名词之后

物主代词要紧跟名词，且在之前