You use he to refer to a man, boy, or male animal.
He could never quite remember all our names.
他绝不可能完全记住我们所有人的名字。
In written English, he is sometimes used to refer to a person without saying whether that person is a man or a woman. Some people dislike this use and prefer to use 'he or she' or 'they'.
The teacher should encourage the child to proceed as far as he can, and when he is stuck, ask for help.
老师应当鼓励孩子尽可能独立思考,自己想不出来了,再寻求帮助。