I absolutely agree - get that 504 in place, and continue to work on understanding how your ds is impacted in school and in testing by his ADHD - that would be my #1 priority; I have two 2e kids, and getting them accommodations and remediation *early* was extremely important; they missed out on some early opportunities for gifted enrichment because of it, but making sure they had their accommodations and remediation and that I understood their challenges well enough to advocate for them made it possible for them to be eligible for and truly benefit from g/t and advanced programs on down the road.
You can work on the g/t side at the same time. No one here is going to be able to explain in detail the next steps of the process in your school district *unless* they are also in the same school district - the process for id'ing g/t kids and the programs vary tremendously in different areas. We can tell you a few things though - the first thought that comes to mind, for me, is that the CogAT is not the most reliable id test for g/t kids. A lot of the EG/PG kids here (including my ds12) did *not* score in the g/t range on the CogAT. It's what can be referred to as a "learned" ability test as opposed to an innate ability test (IQ tests such as the SB and WISC), so school experience/exposure can influence results as well as sometimes EG kids who think way outside the box just aren't going to use the same thought process as expected by the test in answering the questions.
The next thing - you need to understand how your ds' ADHD impacts him in the classroom and in testing situations. Does he need a quiet room? Does he need extended time? Does he need someone to read the questions to him (I think they do this anyway on the CogAT at his age but I'm not 100% sure)? Does he need breaks during testing? Any of the types of accommodations that he needs for regular classroom testing should be included in his 504 plan, and they would then also be included in G/T testing (if they are listed as testing accommodations in his 504 plan).
I would try not to worry too much about the g/t testing at this point in time - let it take place and see what happens. There is usually room for appeal if he doesn't get in, and because his teacher thinks he belongs in the program, I am guessing there's a good chance he'll get in - teacher recommendations often carry a lot of weight. Lastly, if it's a once-per-week pullout and he doesn't get in this first year, he most likely can try again next year and chances are with the typical pull-out programs he will not be behind in any way and he's also not missing something that will negatively impact his life in a huge way. I'm not saying that to say the program isn't worth being in, just that my local experience and the experiences I've heard from other families are that typically once-per-week pullouts are bandaids put in place to make a small effort toward helping g/t kids, but to truly benefit the kids need to be in every-day g/t programs.
Good luck!
polarbear