He'll be called "Mr. President" just like every other former president of the USA, but in order to not cause any confusion they might call him Mr. Former President idk.
This would be an issue if a woman whose husband was not a former president was trying to become the next president, it is pretty clear for now.
President Bill Clinton. That title won't change for him.
If another woman wins in the future, the spouse should still be referred to as just their name. The whole first lady moniker sounds outdated anyways. It's giving a job title to someone that ultimately means nothing.
