... they can, and sometimes do, ask for proof
that a "lap" child is actually under age 2,
especially if he appears older. If you cannot
prove age, they will sometimes require you to
buy the child a ticket. If your child is a little on
the large side, I'd try to be sure that you have
something with you that shows his age.
Southwest is very nice about it, usually, though,
so I don't think they would insist on an actual birth
certificate, as long as you had something relatively
official-looking that had a birthdate on it. Sometimes
health insurance cards have birthdates on them, so
that might work. Otherwise, I'd try to get an unofficial
copy faxed from vital records.