But people who are too old with 70 years of age will actually be better off if they are able to take care of themselves by maintaining their health without thinking about many things because everything must be left up to the next generation like you.
What exactly will his father be thinking about? Mining? How to set up the rig? What type of rig to buy? Op said he gave his grandad investment advice and not how to mine or how to develop Bitcoin so all he has to do is to bring out the money he wants to invest i.e if he truly buys the idea, and allow Op do the investment for him, Op can also be monitoring everything for him and if he sees that profit is good enough he can cash out for his grandad as well so the old man has nothing to worry about, the only thing I would said the man would have worried about is if there are risked involved or not but since Op already told him every about it then that one is settled.