Hello, if my son is going to cross alone with his sisters, does he need any permission?

Depends, if one of your sisters is older, no problem, just your updated passport and visa!

Hi Jessica, A notarized letter is required for a minor to travel to Mexico with someone other than a parent or legal guardian, and is highly recommended when traveling to the US. Additional information can be found here https://help. cbp.gov/s/article/Article-3643?language=en_US

If the sisters are of legal age, you only need their passport documents.

If they are of legal age, not only their documents, whether Mexican or American.