My sister just had a son, and Im kinda freaking out over whether or not Ill be a good uncle. And that whole patronizing adult-thing is exactly what I worry about. How do you avoid that? Just ask the guy "what up dude? good day? see the latest breaking bad? it was messed UP yo." ?
No, it's possible to talk to them like an adult without talking to them about adult things or things that they don't know about. When you see them playing or doing something seem interested. Ask them about their day. If they're playing say something like "What are you playing?", or "That's a cool toy", but not in a super condescending tone like "cool tooyyyyyy" like most people do. When in doubt ask them questions about what they're doing as if you didn't understand. Then when they answer just say "That's pretty cool"