If he's under 2yrs old he can sit on your lap. He will need identification and if older than 2 needs his or hers own seat which you will have to buy a ticket for. Also a car seat for the seat on the plane.
contact you airline
Just take his insurance card if needed also copy of birth records if ssked.
Call your airline
This may be helpful https://www.verywellfamily.com/documents-needed-for-traveling-with-grandchildren-1696241
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