I just tried to add a long, informative post, but my bowser keeps crashing, so I guess I am writing too much.
**note** This is just a quick summary. I do not know all the legal requirements. Other people can give you that. this is just my perspective as a (former) teacher and a parent. I don't agree with how schools always do things, but I am just trying to help you navigate the system as it is.
So here is a shorter version.
504 plans are for students that just need accommodations to be succesful. For you son that might be that the teachers give him a written copy of assignments instead of making him copy from the board, they might email you/him the assignments. He might be allowed to carry his backpack in school so that he doesn't have to keep track of taking things out of his locker. 504 does not require an "educational need" just a "limiting" factor. You will need to give the school documentation of your child's disability from an outside professional.
IEPs are Individual Education Plans based on a student qualifying for special education services. Students must have an "educational need" which at the schools/districts that I have worked at in Texas means that the student has to be failing overall. If the student is making C's and D's that is considered passing and there is no "educational need". With an IEP the student has goals and objectives that are worked and modifications to the curriculum in addition to any accommodations. For your son, that might be that he has goals to improve his organizational skills or social skills. In other words, the school would be teaching him things that they do not work on with no special ed kids.
To qualify for special ed services there is a long process of referral and evaluation. You can not simply give them the outside documentation. This can be very frustrating from the parent (and teacher's) perspective.
You need to make your requests in writing. You will either write a letter to request a meeting for a 504 plan, or request an evaluation for special ed services.
To anyone else reading this, I am not saying that I agree with how public schools handle this. In fact, I don't. Which is one of the reasons that I am not a special ed teacher in public school any longer. I am just trying to help this parent navigate the system as it is currently.