As if if you need a 504 or IEP? You may not know until you have testing completely done. I was surprised yesterday when the nuropsyc evaluation my son suggested we have enough to push a IEP. I had presuming that we would be looking for a 504. In my mind I had seen IEP for issues like Learning Difficulties and 504 for more social difficulties (ADD, ASD). But I guess the difference is really more what kind of accommodations you are asking for with the 504/IEP. The psychologist also said it depends on the school. Some schools one is easier to get than the other.
Good Luck. I like master_of_none's advice that you keep this at getting your DS what HE needs. Write to the school. But keep in mind that they aren't likely to be handing out a 504/IEP the first week of school. There are policies and procedures and these take time. School have to start the process, try obvious things first and then usually show they aren't working before they will even start testing.
