The boy went off,a pinwheel in his hand.

很高兴为你解答.
保证正确率~!
【解析】：
其实这三种表达都是正确的：
a pinwheel in his hand = pinwheel in hand = with a pinwheel in his hand
有 with 的是 with 的复合结构,没有的即是独立主格结构.其实本质上,是没有太大的区别的.
pinwheel in hand 是 a pinwheel in his hand 的便写形式,即是省略了冠词和形容词性物主代词.
因为它们这个动作的发出者很明确,就是主句的主语,所以是可以这样省略的.
【翻译】：
那个男孩走了,手里拿着一个纸风车.
祝开心~!