First of all it is very hard long term to have a good relationship with a bus. partner.
You have choices:
1. Take him out for a beer and discuss his recent purchase of the high miles Ford and what it takes to fix it. Ask him who is paying for it? Who is fixing it? Put the ball in his court. Rub his nose in it like a dog with poo.
2. If he admits he has been an idiot there is hope yet. Next time tell him to consult you before major purchases.
3. If he goes into a rage(I doubt it because you love him like a brother), consider buying him out because his kind of thinking and actions can become worse.
They say timing is everything. Communication is huge too in terms of a big decision in a partnership.