You should encourage him to it因为这里encouragement是名词,这里需要的是动词,所以把名词改为动词就好了.
It's sad of him and equal.在It's的用法中,如果这个形容词主要修饰是人,用of,修饰的是一件事,就用for.
比如：
It's kind of you!你真好,这个很明显是修饰人,一般这种词关于人的喜怒哀乐好坏的都用of.
It's importent for you to study hard.这里是说努力学习这件事很重要