It seems to be Alaska Airline's favorite question! We get asked going to Seattle, we get asked going to Canada. I carry a letter signed by my husband that I have permission to travel solo with our son, including taking him out of the country. Maybe legally they can't insist on this for domestic flights, but I have been asked, both at check-in and by the gate agent. I prefer to avoid extended arguments, so I carry the letter.

Some countries, wishing not to get in the middle of child custody fights, demand this type of parental permission before allowing in a solo adult w/ minor child - I think Mexico is one, not sure about Canada.

p.s. son and I have different last names, if it matters