Your manager's job is to manage you. You want to tell your manager that you will leave if he tries. This is going to be an adversarial discussion no matter what. If you want to get the point across without giving him an ultimatum, first you need to understand his reasoning, and then you need to gently explain yours. Whatever he says, respect his authority as manager and end the conversation on good terms.

##Understand his Reasoning

"Try to honestly see things from the other person's point of view."

- Dale Carnegie, How to Win Friends and Influence People

You suggest that this new manager is only talking about the eight hour day because he has never worked with developers before. Are you sure? The old manager could have been removed because he didn't enforce the rules. Maybe upper management noticed people were constantly out of the office and asked the new manager to improve on that. Maybe he is worried about being short-staffed because other developers are leaving.

Each one of these are a very different problem from his perspective, and require different finesse to work around on your side. Rather than going in and laying down an ultimatum, the first thing you should do is to ask him what his reasoning is. For instance:

Hey boss, I wanted to follow-up on that conversation we had about working hours. Since joining the company we've tended to have flexible working hours and encouraged to have a healthy work-life balance so long as the job gets done on time. Since we're going to be working together, I want to try to understand your goals for managing me that way. Would you mind explaining the change?

The point is to say, "This feels kind of sudden, so as a mature adult I want to understand it" rather than to go on the offensive. This gives him a chance to think about it, explain, and maybe even shock you by having a good answer.

##Gently Explain your Opinion

"Don't criticize, condemn or complain."

- Dale Carnegie, How to Win Friends and Influence People

You may disagree with his reasoning. You may think it's stupid and will make everyone quit. And it may. But telling him that directly won't do anything but make him upset. You need to gently explain your position, for instance:

Part of the reason I've been so loyal to this job and this company is the flexibility it allows me. I understand you want me to be here 8 hours a day because <reason x>, is there any way to find a way to satisfy what you need, and still allow me some flexibility when I have things I need to take care of during work hours?

By looking for a way to compromise, and asking him to lead you to the answer, he will be far more willing to give you some leeway rather than if you just demand it regardless of what he wants.

##Respect his Authority

At the end of the day he is the manager and in charge of making these decisions regardless of whether or not you agree with them. Respect that.

If he compromises and lets you have a bit of flexibility, say, "Thank you very much, I look forward to working with you."

If he doesn't compromise at all and tells you to work 8 hours a day and take vacation time if you have errands, say, "Thank you very much, I look forward to working with you."

Issuing an ultimatum will not change his mind. If you really can't live with his decision, start interviewing elsewhere, and issue your letter of resignation. Threatening to quit won't make your work life any more pleasant (it will just make matters worse), so better to quit on good terms. By negotiating with your feet you will get much better results than butting heads with your manager.