> Unless anyone else has any suggestions or strong objections.
Go with your guts feeling. When I was a kid, I always call my classmates' grandparents the same way my classmates call them, even though I met them for the first time. And that's also how I addressed my then-girlfriend's grandparents. You can call someone 老伯伯 or 老先生, etc., when you have completely no relationship (not even through a third party) with him. At the end, the important thing is whether your boyfriend wishes to present himself as a complete stranger or as a future family member to your grandparent.
Go with your guts feeling. When I was a kid, I always call my classmates' grandparents the same way my classmates call them, even though I met them for the first time. And that's also how I addressed my then-girlfriend's grandparents. You can call someone 老伯伯 or 老先生, etc., when you have completely no relationship (not even through a third party) with him. At the end, the important thing is whether your boyfriend wishes to present himself as a complete stranger or as a future family member to your grandparent.