I don't think you need an IEP. I think you need a 504 plan.
An IEP is an Individualized Education Plan, and it is generally intended for students who cannot learn or be taught in the traditional way. They have special needs, or intellectual disabilities, and it usually involves a basic inability to learn or participate along with the rest of their peers.
A 504 plan is intended to accommodate a student who can learn in the traditional way, but needs a modification in the classroom. Some examples would be: a diabetic student who is allowed to leave the class at any time for insulin or blood testing or food; a student who uses hearing aids and needs to sit near the teacher in order to hear better; a student who is unable to use his or her arms and cannot take notes and therefore must have a person take notes for him or her; a student who has dyslexia and needs an audio recording of the class. My dd has dysgraphia and is unable to write on the lines or write legibly and was allowed to use a keyboard device instead of handwriting anything. Like your son, her sentence construction, spelling and reading were levels above other students, but no one could even begin to decipher her writing and it didn't follow any lines but slanted way off the page.
The 504 plan is national, and is so named because it is section 504 of the Americans with Disabilities Act.
An IR&S could follow the establishment of the 504 plan. The school staff would say "ok, now that the 504 plan is in place, and it is established that your son cannot use a pen or pencil to write, we will now discuss how to accommodate his needs" and at that point they would say "let's get him a keyboard" or "we will have an aide write for him" or whatever is best. It's a way of identifying resources and support so that the 504 plan can be implemented best.
I would suggest you gather your professional evaluation records, and get a statement from a supportive doctor. Have the doctor state very very briefly "_______ has been diagnosed with hypotonia. His condition prevents him from writing legibly. He requires the use of [whatever you decide, like a keyboard or a scribe] and requires that a 504 plan be established for him." Have the doctor specify that this is not a short term problem resulting from, say, breaking his arm or something, but a long term condition that will require therapy. The briefer the doctor's note, the better. If necessary you can go into detail later.
A scribe, by the way, is what a school calls a person who either takes notes for a student or sits beside the student and fills in the answers on a test like the ACT or the state assessment test, or who reads the quiz aloud to a student and then records in writing what the student orally answers.
If your son's school will not comply and establish a 504 plan for him they must inform you of your rights. A parent cannot insist on a 504 alone - the school must recognize that the student does have a need for accommodation or a physician must inform the school.
Here's an article that I think is really clear and helpful:
http://www.greatschools.org/special-education/legal-right...
By the way, there will be regular meetings to update the 504 plan, like at the beginning and end of the school year, and if anything changes, you should put it in writing to the school. My dd's 504 plan keeps getting updated as we receive clearer diagnoses (she has medical needs in addition to the dysgraphia). Start thinking about how you think your son would do better in school, and keep notes. For example, do you think he needs to use a keyboard, or special pens, or does he get tired from trying to write and needs to have a reduced assignment (if the teacher says "write a paragraph of at least 6 sentences" would he do better if he only had to write 3 sentences), etc?
Also, if your son can use a keyboard, this is a great tool for kids: http://www.neo-direct.com
I hope this is helpful to you.