How would putting a contract in writing cause your brother to lose financial control ???

Contract only spells out what each person's resposibilities and rewards are. (and whatever else they write up/spell out in the contract.)

No  written contract, and "he does all the books",,,???   you can bet your sweet bippy I would be demanding to see and have every stinking month's income and expenses shown and explained.

I doubt I'm wrong,,,,,The real reason your brother doesn't want anything is writing is he will actually have to follow what ever 'rules' are set forth, and can't change things as time progresses..