First time I had to fire someone the CEO of the company I worked for gave me some advice in line with this.

Basically start with "I have some bad news for you. The decision has been made that today (or whatever date) will be your last day here."

If this is just a basic workforce reduction and not due to anything this particular worker has done, then you might include "The company is undergoing a reduction of workforce, and the decision has been made..." You get the idea.

Then tell them about the severance they'll be getting. Have someone from HR there as a witness to the conversation, and then tell the guy that HR can answer any questions he has about the specifics of the severance, insurance, or any other questions he has about benefits, etc.

He suggested that you say "the decision has been made" in order to cut off any efforts to debate or get you to change your mind. If there's push back or attempts to negotiate, the reply is "I'm sorry, the decision has already been made."