it is not his parents,as I see it,that influence him so much

that引导定语从句,用来修饰parents.
It is not his parents.it 是形式主语,is not 是be 动词,his parents 是表语.
As I see it是插入语,不影响整个句子的任何结构.
整个句子与his parents don't influence him so much.比,强调了his parents.