1 I'm going to have a meeting with Fred: formal.
2 I'm going to meet Fred; informal
If you and Fred are colleagues and also friends, think about why you are meeting: if it is to do work, it is at the office, your boss is entitled to know what you discuss, 1. If it is private, even if you just spend your time talking about your colleagues, 2.
If you're going for a drink, you would probably just say you were going for a drink with him If you were meeting just to talk about work, you'd say you were going to talk about work with him, or maybe you were going for a drink to talk about work, etc.