So frightened was he that he did not dare to move an inch.
谓语was放在主语he前面?

“So + 做表语形容词” 置于句首，主句看上似乎是完全倒装了，这也许因为be动词的特殊性。但在语法范畴，还是部分倒装。

【下面是一些摘录的语法规则，供参考】
在so... that…结构中，若将so+adj. / adv. 置于句首，则其后的主句要用部分倒装。如：

So cold was the weather that we had to stay at home. 天气太冷，我们只好呆在家里。

So much did they eat that they could not move for the next hour. 他们吃得太多了，一个小时都动弹不得。

So loudly did he speak that even people in the next room could hear him. 他讲话声音那么大，隔壁屋里的人都听得见。

So fast does light travel that we can hardly imagine its speed. 光速很快，我们几乎没法像它的速度。

So sudden was the attack that we had no time to escape. 袭击来得非常突然，我们来不及逃跑。

类似地，当such…that…结构的such…置于句首时，such后的句子也要使用部分倒装。如：

Such a nice man did he seem that we all believe him. 他像个很和蔼的人，所以我们都喜欢他。