My mother has the guardianship of my nephew in Brazil and…

As neither parent would be in Canada so long as you were the legal guardian of the child the child could attend school without a study permit.

They would need a visitor's visa.

And you are right and you would have to pay international student fees.

You would need proof that you have legal guardianship as well.

Does that help?

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That is the tricky party.

It is not true that he can only visit for 6 months. It is quite possible that the CIC would issue a visitor's visa for a least the full school year.

If he were to get a study permit he would need a visitor's visa in any event.

CIC has conflicting information on their site and it is never clear what they expect for minor children who are not with their parents.

There are legal forms for you to fill out and you should read all of this page:

https://www.canada.ca/en/immigration-refugees-citizenship/services/study-canada/study-permit/prepare/minor-children.html

http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=488&top=15

Perhaps it is safest to apply for the permit and see if you can get it.