This is a common problem faced by a every one working in helpdesk.

1st thing don't try to argue with him.

2ndly try to understand the issues or problems he is facing.

3rdly don't promise him heaven and earth.

Finally keep a cool head don't concentrate on what he is saying and just concentrate on the solution.

As a help desk professional your duty is to focus on the best possible solution.

Don't get carried away.