My IPhone warranty

You can see if the iPhone is eligible for AppleCare warranty by checking in the Settings app. Go to Settings > General > About, and check under the “Coverage” menu. You should see your warranty info there; probably a 1-year warranty depending on how long ago your boyfriend purchased it. But the point of this is to check and see if your phone is still eligible for AppleCare. If it is, you can buy it (and I highly recommend you do, it’s definitely worth it.)

You can find all warranty detail here:

Legal - Hardware Warranties - Apple

Note that if he bought it commercially in Japan (e.g. from an Apple Store), it generally only has a Japanese warranty.

The exception is that if he purchased it from an overseas U.S. military exchange, a U.S. warranty would apply.