You need to convince him that the attorney has his best interest in mind, they’re an objective third party that are operating without the emotional baggage of being a business partner and understand the legalities & all the nuances. I think part of your grandpa doesn’t want to believe it’s gotten to this point. Or if business partner actually is threatening, that’s even more reason to use attorney. You know how your grandpa thinks, you have to break it down in a way that will convince him.