In a positive sentence we use:
He does his homework everyday.
Why with the modal can does it then change to DO with he & She?
He can do his homework everyday if he wants to.
The infinitive, the basic form of the verb, is 'to do'. When verbs are used in isolation, they are inflected:
When we include a modal verb, the form is, modal +infinitive
I can do
He can do
they can do
we can do