G.♣.
That sounds so overwhelming. You must really be beside yourself with worry. I know I would be.
There isn't a whole lot you can do right now. I'm sure that is very frustrating, but I hope you can take some comfort knowing that at your appointment you will be speaking with the person who actually does know what is going on with your son and what the next step is.
My boys have both had to see specialists at different times. There is really nothing I could do to prepare unless it was to fill out a questionnaire before hand. We have been very blessed with doctors who take their time with us, really try to be very clear with us and are willing to answer questions.
One thing you can do is keep a notebook or pad of paper with you and just write things down as you think of them. They could questions, observations, whatever. Anything that pops into your head, write it down. Bring that with you to the appointment. Before the doctor leaves, be sure to look over everything you've written down and make sure there isn't anything else you'd like to mention.
Other than that, try (even though it will be very hard) to simply be and enjoy your time with your. It's only natural to worry, but do your best to just be present to your son and your family. Right now, you are still waiting for answers. When you do see the doctor, you will begin to deal with whatever you need to deal with. Until then, the only thing you can do is write down notes, and live your life.
Wishing you peace as you wait for your appointment.