Hi Cosmo - I've no experience with 6 year olds so not sure what you should tell him about the appt. My DS is 3.10 and has very limited understanding so I didn't tell him anything about the appt. as there wouldn't have been any point. I think if I were you I would just say it was a Doctor's appt.
Our first appt. with the paed. lasted for about 90 minutes and started with her observing DS in the waiting room. She wanted to see how he interacted with family and whether he made any attempt to engage with the other children.
Although we were there because our H.V had refered DS she wanted to hear about DS's problems from our POV. At this stage she ws only interested in where he was WRT development right now, and not about baby years or my pregnancy. She also asked about relatives with developmental issues/learning difficulties. During this DS was given puzzles to keep him busy, but she was watching him all the time. Then she gave him some informal tests to do. She timed him with shape sorters and puzzles so she could see if his fine motor skills were age appropriate. There were also the normal tests that H.V do such as build a bridge with 3 blocks and draw a circle. At the very end she weighed him, mesured height and did a brief physical which involved genitals.
At the next appt. we were asked about DS's developmental history and my pregnancy. Then she gave her DX.
I'm sure it's different for everyone but I've never heard anyone say it had been a negative experience.
As has already been said, I would write down any worries that you have and any questions that you would like answered. I did this but didn't really find it necessary on the day because the Paed. was so thorough with her questions.
Hope it goes well for you both. Good luck