He speaks English most fluently in his class.

你这个问题涉及到比较级和最高级人或事物的比较的知识.
两者之间用比较级,而三者或三者以上用最高级. He speaks English most fluently in his class意思是他是他们班上说英语最流利的人.这里是三者以上的了,所以用了最高级most.
而Nobody else in his class can speck English more fluently than he."意思是没有人能比他说的再好不过了".是把nobody else看成一个整体,和他进行比较,因此是比较级.所以使用more.. than..例句:A compute is more expensive than a bicycle.