Leave the job and take it over
The business he built from nothing stays in the family, and he hands it to someone he trusts rather than watching it go to a buyer.
If you do this: Your career path at the big company closes or pauses; your days become the shop's days. Your father gets continuity and relief; your brother stays out of it.
Cost: A career going well, and years before you could return to it; income roughly the same.
You: Tell your father you are seriously considering it and ask for three months of shadowing him before you resign.
The exact first sentence“Papa, I am willing to consider taking it on, but I want three months learning it beside you first.”