He has to help his mother clean the house today 改为否定句He ------ ------- help his mother clean the house today.

He 【has to 必须；不得不】 help his mother clean the house today 改为否定句
He 【need not】 help his mother clean the house today。
=He 【doesn't have to 】help his mother clean the house today。

祝你开心如意！

He has to help his mother clean the house today 改为否定句
He【need】【not】 help his mother clean the house today。(情态动词使用+not+动词原形）
He【doesn't】【have】【to】 help his mother clean the house today。
He【doesn't】【need】【to】 help his mother clean the house today。(行为动词使用+to +动词原形）

has体现三单,所以借助助动词does进行否定.
正确：doesn't have to

He has to help his mother clean the house today 改为否定句
He 【need】【not】help his mother clean the house today。
need not do sth 没必要做某事。这里的need是情态动词，不要加s。