I think I mentioned this before, but my significant other lives in the United States, and I live in Canada.
The plan (that I'm just starting to work on) is for us to live together long enough to be considered common-law, then I will sponsor him as a permanent resident.

I know that Americans can stay as visitors for 6 months. How easy is it to get this renewed? I'm hoping he can get it renewed so he can stay 12 months.

You can certainly apply and most likely it will be approved, he may not get 6 more months however. Also make sure and apply early, don't wait till the last few weeks or you may not get an answer in time.

Below is a link for how to apply for the extension

Temporary resident visa: Extending your stay