Any suggestions on how to re-approach this with our father

Just one. Don't.

This no doubt sounds strange coming from one of the biggest nags about estate planning, but having been to hell and back trying to get my 90 year old father to stop driving, I know the type you're dealing with. He's not going to change, so the only behavior you can modify is your own.

The good news is that, as has been previously explained, you have no tax or legal issues at the moment. Your lives will be more complicated than necessary when he dies or becomes incapacitated, but people get through it all the time.

You might want to consult a lawyer on your own to discuss durable power of attorney issues. AFAIK since his accounts are in joint tenancy there's nothing to stop you and your brother from managing his funds if he becomes incapacitated, but getting some legal assurance on that ahead of time would be wise.

If you haven't done so already, you should approach your father about living will/health care power of attorney issues. If the time comes that you have to speak for him, you'll feel much better knowing that you're doing what he wanted.

On a personal note, enjoy the ride. It sounds like you're dealing with a character, and that can be a joy and a curse. I used to calm myself with "I know the day will come that I'd give anything to have him aggravate me one more time." Now that the day is here, I'm glad that I concentrated on the good.

Phil