How do I force my dad to actually let go of the day-to-day business?

Send him on a month-long unplugged trip with the spouse, no laptop, no phone. Kris Kluver, in The Dysfunctional Family Office, calls this the Forcing Device. The Mitchell family used Cayman. Robert and Joanne went for one full month, fully disconnected. The trip forces what the calendar alone won't. The founder physically can't intervene. The team has to handle decisions without him. The new CEO actually gets to lead. The company discovers it can run without the founder hovering. By the time the founder returns, the new dynamic has set. Most founders won't take this trip voluntarily. They have to be pushed into it by family agreement, scheduled in advance, and treated as non-negotiable. The Forcing Device works because it removes the founder's option to drift back into running things. The discomfort during the trip is part of the point.