FAPE applies to every child in this country. The presumption is that every one is to be provided a
Free and
Appropriate
Public
Education. It is the standard against which a given child's education is supposed to be measured.
While the school district may insist that they can "only" consider a disability if it can be measured on a standardized test that shows 1.5 grade levels delay or 1.5 standard deviations below the mean - they are wrong and/or lying to you.
There are a few states that have specifically legislated some rules about determining specific areas of disability - usually speech and language it seems. But federal law trumps state law when it comes to the provision of FAPE which means that those state laws can be legally challenged if they prevent a child from preceiving FAPE.
IDEA has always said that an IEP team has the power to decide a child is
1) a child with a disability and
2) NOT receiving FAPE
and therefore qualifies for an IEP regardless of any testing or common practice or grades. The law requires that the IEP team documents how the team decided the child was disabled in the IEP but that's it. The IEP team is not legally bound to any rules of any kind - if the team can agree on the disability and if they can provide a written justification for their decision then they can determine a child disabled and create an IEP.
But you will never hear any school official tell you this because it would be chaos if there were absolutely no guidelines to follow when deciding which children to assess. Unfortunately, many school districts see these as "rules" and you must force their hand by requesting assessment in writing and insisting they either assess or issue "prior written notice" - which is simply a letter telling you why they believe your child is not a child with a disability and should not be assessed.
As Insane has suggested, many kids "age into" their disabilities at key points in the academic process. At 4th grade, 7th grade and 9th grade the academic demands take a giant step forward. Kids who could get by up until then even if not doing well - suddenly they are floundering because they cannot keep up with the greater cognitive, social or physical demands (or all three).
It certainly sounds like more is going on that ADHD. As Insane said, many kids have language processing problems that really interfere with their ability to process language but you don't notice anything obviously wrong when you talk to them or when they are asked to do easy tasks not under pressure.
I suggest you get an audiological hearing test done (one where they use the sound proof booth) to make sure your son's physical ability to hear is good - this is always the first thing that should be done when there seems to be a language processing issue. it is not Central Auditory Processing Disorder (CAPD) testing but rather just ruling out hearing loss. Even mild hearing loss in one ear can lead to academic problems so it is important to do this and not rely on the school's very primitive hearing tests or the screening ones done by pediatricians.
If he is struggling that much and his teachers know it, the school should have called for a 504 meeting to review the 504 and consider whether he needs additional supports and/or Special Education. At the minimum you need to call for a 504 meeting to be held in the next couple of weeks.
Don't wait for that though. If I were you I would write to the school and document your concerns, the degree to which you are helping your child at home and request that he be assessed for Special Education. if you request assessment now in early October you may get an IEP in place by March. If they call and tell you "oh no he has a 504" just say no, I'd like him assessed for Special Education. Do not argue with them and do not agree. Just be clear and firm.
It's important to document the amount of help he is getting at home because this is a factor that the IEP team is supposed to consider when deciding if he is a child with a disability for the purposes of an IEP. The fact that he is still struggling despite hours of help from you is strong evidence that he is not getting FAPE with his current placement. "Placement" is not a physical place. It is the combination of services, supports and modifications that are described in an IEP and that the school is legally required to provide in order to ensure that a disabled child is getting FAPE.
I suggest you spend your free time reading up on 504's and IEP's on
www.wrightslaw.org.