It is important for him to learn to read.
这是用It 的固定句型结构之一,其中,It作形式主语,真正的主语是【for him to learn to read】,这是【不定式的复合结构】：介词+for/of sb +不定式.
原句可以转换为：
For him to learn to read is important .
而这种结构在英语中习惯上用It作形式主语.再如：
It is kind of you to help me.