You are in a very common grey zone. The school is probably not going to help you, and when the ECI folks looked at him in preschool, he did not qualify for services, but that did not mean that he did not need therapy. This is a clasic reason that I always tell parents that public services are only there to make your child functional, and for you son, he is fuctional in the classroom, as far as the school is concerned if he is passing and not causing a problem for the teachers (meaning behavior, but also if they expect that he will pass the state mandaded assessments or not in the 3rd grade, and my guess is, they expect he will pass those.)
As I said though, that does not mean that he does not need therapy to maximize his potential. Your first step should be to first, stop asking what it could be with a specific name like dyslexia, and second, call a Neuropsychologist and have an evaluation becuase of all the reasons you listed, not to rule something in, or something out, but to ask the professional exactly what it is, and if that is not possible (sometimes, there is no word for it) what you need to do to help him be all that he can be.
I would also suggest that you call the nearest childrens hospital, and ask the OT department for a referal to a Developmental Optomitrist, not just to check visual acuity, but also to check on the development of his visual skills. This would include visual motor, visual perceptual, tracking, and the like. How he uses his eyes is important. While you are on the phone with the OT department, make an appointment for him to have an evaluation for his handwriting, he could benefit from therapy to help him with a more effecient pencil grip, and thus, a more fluent writer. My guess is, he will be overwhelmed when school takes a turn to reading and writing to learn, instead of learning to read and write, and if you think he is resistent to writing things down now, just wait, if you do not intervene, he will become more resistant, and unfortunately, you may have to provide the lions share of his therapy.
Basicly, you play right into the school districts strong suit by asking if your child has a particular disorder, especially the one you picked. Dylexia is the inablity to manipulate phonemes, and his spelling ablity is just too good for his issue to be a pure dyslexia, though he may have related issues, the school district will not see them as they have a very narrow view of what to look for, and it is based on excluding as many kids as they can. They are under no obligation (unless you make them by using the magic words) to say, no, I don't think he has dyslexia, but it may be something else, especially if he will meet the minimum standards. The magic words are those that you can learn at www.wrightslaw.com, which is the parent advocates bible. There is a lot to read, but I would start with basic things about evaluations, how to get an evaluation, how to ask for an evaluation, and the extremely important need to own a private evaluation so that you never know less about your son than any public agency.
Start reading at wrightslaw, get an appointment with a nueropsycholgist, a developmental optomitrist, and an OT. After you have the neurpsychological, see a medical professional if they recomend it, either a developmental pediatrican or a board certified child psychiatrist. If you feel overwhelmed by the school information, seek out an educational advocate, you can find one in your area on the wrightslaw yellow pages for your state, on the wrightslaw web site. Remember, your son must have both a qualifying disablity (or you must suspsect that he has one) AND he must have an educational need for special education to get services, and the educational need is going to be the thing that trips you up for a child who is very high functioning and has extreem aptitude in something like math.
Good luck,
M.