How do I convince my grandpa to replace his swollen laptop battery?

Tell him to do so r you'll start planning his funeral, no joke it is about to kill.

Assuming they let him onto the plane, I won't if I am airport security.

Usually you can get new batteries for pretty cheap... I'd say just get it replaced.

i doubt they will let him fly with the laptop and he might even be detained because he knowingly tried to bring an explosive/incendiary device on the plane (bad batteries explode/burn all the time)

Look up the replacement battery online and order it yourself without telling him that you are doing it. Then tell him you are going to replace it because you know airport security will confiscate it and you don't want him to lose his laptop. There's no convincing stubborn old men, they only understand deliberate and decisive action.

I suppose starting a fire in an aircraft with dozens on board is not a big deal for an old guy like him with no tech knowledge.

I guess tell him, replace the battery or replace the laptop (which would eventually be the case if that keeps up). Money is involved either way but of course replacing the laptop would be more than its battery.

Show him YouTube videos of laptop batteries that go bad and blow up

it's probably a fire hazard. why don't you buy him a new one