A 504 plan is based on the Americans with Disabilities Act. It is enforceable by filing a lawsuit in federal court. This is a significant barrier to most families and so makes a 504 plan difficult to legally enforce. It is possible to do but much harder than an IEP. If you feel that a 504 plan, when followed, is all your son really needs at this point, you may want to contact your SD's superintendents office and ask to speak to someone there about your difficulty. The SD may have a district wide 504 plan coordinator who can help you at the local school level. But I wouldn't count on it.
An IEP is created by the IDEA 2004 legislation that mandates help for students with disabilities whose disabilities are negatively affecting their ability to benefit from a free, appropriate public education (better known as FAPE). Issues of eligibility for special education and disagreements between parents and SD's over the IEP are adjudicated through a separate system than the regular courts that is much more accessible to parents. It includes the option for mediation, procedural violation complaints and due process adjudication for eligibility and content disagreements.
The purpose of a 504 and an IEP are different. A 504 plan is intended to "level the playing field" by removing obstacles to a student's access to the regular curriculum that only occur because of the student's disability. For example, ramping a staircase makes a classroom accessible to a student in a wheelchair. In your son's case it may be that his 504 plan calls for him to take a set of books home or that he get study guides to help compensate for his exec function issues. There is no actual modification of the general curriculum or "special" instruction/services required for him to access and presumably benefit from the typical FAPE.
An IEP is an individualized educational plan that is created because the students' disabilities make it difficult or impossible for the student to access or benefit from the regular classroom instruction without modifications to the regular curriculum or the addition of related services like speech. Your brief description of your son's issues suggest that he might qualify for an IEP and that he would certainly benefit from a 504 plan. And IEP trumps a 504 plan every time and there is no need to have both. But I would not abandon th 504 plan until a satisfactory IEP is in place.
It is not unusual for a student to "grow" into their disabilities. In other words, as the child rises through the grades the academic demands grow as well as the expectations for emotional control/maturity. A child who was able to get by before then often seems to suddenly be struggling and it is typical for this to happen during the 4-6th grade years. That's when there is a big shift to reliance on reading, taking notes and being able to learn and function independently (compared to the primary grades).
It sounds like your son may be experiencing academic decline at this point. Is this correct? Has his teacher told you that he will have poor grades or that he is not performing at grade level? Did he do poorly on last year's state-wide standardized testing? These are not the only markers to look for but are certainly the most obvious.
Even if he is not "failing" he may qualify for an IEP due to deficits in the social realm or due to behavioral issues like not turning in completed work. If you are providing extensive help with homework at home or paying for tutoring in order to keep him at or close to grade level this can also be considered when the IEP team meets to decide eligility since without that extra help he would probably be failing.
I would go ahead and request that he be evaluated for special education in writing. There are a variety of ways to do this but my suggestion would be to write a simple letter to the principal, stating that your son has the following diagnoses and you are concerned that, due to his disabilities, he is having trouble at school. For this reason you would like to have him evaluated for Special Education. I would also suggest hand carrying your note to the school principal and delivering it in person (make an appointment). Make some notes about the time/day of the meeting, what he/she was wearing, etc. in case you need to "prove" that the letter was delivered. Or you can send it certified mail in addition to hand delivering.
At that meeting, if you haven't already talked to the principal, I would let him/her know that your son is losing recess daily as punishment for issues that you feel are disability-related. You might want to bring copies of your e-mails to the teacher showing the date and content to give to the principal to show that this teacher is failing to respond with even the minimum of courtesy to your concerns.
This may result in some immediate changes in the classroom to adhere more closely to the existing 504 plan. It may not too - depends on the principal.
Once you send them a written request for evaluation, the SD must respond within 60 days by either sending you a proposed assessment plan or by denying your request and giving you specific reasons why they feel your son does not need special education or related services. The latter is referred to as prior notice.
Given your son's documented diagnoses, I cannot imagine the SD refusing to assess but it may happen anyway. They are supposed to assess all children when a disability is "suspected". If they deny your request then you will probably have to file due process. If you get a denial, post about it in this forum and you will get suggestions about how to proceed from there.
If they agree to assess him, they should send you some kind of written assessment plan for your approval. I would request assessment in every single area of potential disability including speech, Occupational Therapist (OT), behavioral, academic - the whole shebang. A lot of districts have a form where you check boxes to indicate the areas of assessment. Check any that the SD hasn't checked that you even remotely suspect might apply. You are allowed to do so - don't let the SD tell you you can't.
On the form or letter write in that you require that copies of all reports and findings be provided to you at least 5 working days prior to any meeting to determine eligibility. Make a copy for your records.
When you return the signed permission to assess I would also send them copies of all the reports you have to date from his doctors/specialists so that they have them for his records.
I would not be surprised if your son has speech deficits that rise to the level of qualifying him as a speech/language impaired student. Once a student qualifies for special education under any one classification then related disabilities should also be addressed by the IEP team.
Good luck