There's always more to these posts than any of us can sort through. Your use of "we" gave me concerns, especially when it often translates to the younger generation donating years of their lives to a cause that crashes out and eventually kicks them to the curb. Make sure any "we" involves a return to you, the individual.

You definitely need to have the talk with your grandfather. "Where do I fit in the future of this operation?" shouldn't be seen as any kind of conflict or problem. It's something you need to know. If the conversation goes south or it eventually becomes obvious you're just an employee, you need to start making some adjustments. Keep in mind his decisions are temporary unless he takes the steps to make them permanent. Don't fall in love with "one day it'll all be yours". There's an endless list of reasons why that probably won't happen and they're all out of your control.

If there is pushback at any point along the way, I would suggest looking into a community college, a trade school, or just finding a job that will let you get out and see some of the world. I'd give them a warning that it's going to happen and maybe try to time it so the transition is easy for them, and then just make it happen. Whether the place comes to a screeching halt or they hire a replacement, just let it happen. Don't hang around and let a sinking ship bound for the scrap yard rob you of the best years of your life when you could be on your way to building your own.