Can a doctor render someone incapacitated?

I imagine he must not have Power of Attorney in place?

"rendered incapacitated" -- do you mean declared incompetent?

Who is the conservator? The doctor alone couldn't do it. It would have to be done through the probate court. There would have to be evidence that the person was incompetent to handle their own affairs. Who is the one who filed for conservatorship? Was your father spending money too easily or were there people taking advantage of him?