You seem to have two questions. First, what is next? A full evaluation is next. Do your son a favor, and keep your feelings from the evaluator and just answer the questions and tell them what you see without trying to guide them into seeing what is most palatable to you. If your son gets a diagnosis that is based on what you want him to have, you may not give him the help that he needs.
Second, what can you expect from an evaluation? You should expect that this professional will assess medical, psychological, nuerological, educational, occupational, speech and langauge, and behavioral aspects of your sons functioning, and give you data refelcted in standard scores that are compared to a very large "normative" sample of children his age. That professional should write a long report that will spell out all his standard scores and will spell out something called subtests of each evaluation they do that will explain nuances in his processing and functioning that will both assist them in a diagnosis and provide direction for theraputic intervention. The last part of the evaluation should give you a treatment plan that will include many therapies based on the data and a recomendation for medication trial and care, if appropriate for his functioning and condition.
You should expect that this evaluation will take many hours. If it does not, look into a new evaluation from a Developmental Pediatrician, who will provide the most comprehensive evaluation you will find. You can also get a pretty good evaluation from a nueropsychologist combined with treatment from a psychiatrist, but you will have to supplement with ancellary therapists like speech and OT.
BTW, ADHD is very often (more than not) comorbid with sensory issues so there is nothing about what you said that rules out ADHD in reality. Food allegies and sensativities are best dealt with by an allergist, which is usually not part of a nuerobiological evaluation or the intervention that is recomended for children with them, though there are alternative practioners who beleive this and will use diet as a first round intervention. If you think your son has allergies, seek out an allergist, but I am afraid that you will be disapointed with a neurodevelopmental evaluation in that regaurd, and you should not expect that this will be a significant player in either the diagnosis or treatment. You can be sure though, that if there were significant evidence that diet and food sensativity were involved, this aspect would both be a standard aspect of these evaluations and treatment for issues like your son is experiencing. There is no conspiracy by the medical community to eschew any treatment that would help kids like your son, dispite popular myth that pysicians would rather "hand out pills like candy" which is just rediculous.
Treatment is far, far, far, more than "just" a pill, and frankly, even if it is not ADHD, and it might not be, the treatment plan will likely look very similar to what I about to describe because therapy is targeted to what the data indicates, not what the diagnoisis is anyway. Medication, when appropriate, is a tool used to help children with issues get full benefit from cognitive behavioral therapy, play therapy, social skills classes, Occupational therapy, speech and langauge therapy, pysical therapy, educational interventions, and behavior plans, which are going to be the meat and potatoes of his treatment no matter what he has. You may find yourself with many hours a week of appointments for your son's treatment, and then homework for each, so appropriate treatment is way more than "just" a pill. You will grow to appreciate how important that pill is if it shortens the number of months you drive him to his therapies because your son is getting more from each one and can attend to the coping skills he is learning for self regulation more readily.
It sounds to me like your school wants to offer your son services, but they need a medical evaluation that says ADHD to do so. Do not be afraid of that. Services at school are not based on a diagnosis either, so it is useless to argue with the school about what to call him, and frankly, the school may have room in their numbers game with the state to provide what your son needs in that category, but as I said, services are provided based on his needs, not what you call his condition or how they qualify him for services. Once you get to the point where the school is offering services, you should have your own evaluation, undertand the data so that you know what he needs, and then advocate that he gets it, based on the data. His diagnosis is of no concequence expecpt that the school may need a medical professional to sign that he has a medical condition so that they can offer him a needed service. www.wrightslaw.com will be a good source for you to understand how to advocate well.
A good start on understanding these issues is a book called "All Kinds of Minds" by Dr. Mel Levine. His expertise is not limited to ADHD, but he does explain it and all the other similar and related issues.
M.
Added:Ritalin is a stimulant, not a narcotic, and it is not the only medication available. The idea that ritalin is a narcotic medication is horrible misinformation. Stimulant medications work because of the way brain cells function, they don't touch and for our "thoughts" to move over the space between brain cells (called a synapes) our bodies produce electrochemicals to carry that thought as an electrical impulse. People who have damaged nuerotransimittler receptors or who do not produce eneough of the neuroransmitters cannot rely on their brains to function properly, and they have ADHD. Stimulant medication, NOT NARCOTICS help this problem, and there are other medications that are not stimulants that adjust other neurotransmitters as well. There is zero use of narcotic medication for ADHD, and this thought stems from the very mistaken and antiquated idea that children with ADHD need to be calmed, which is really opposite of what they need. ADHD medication does not cause "zombies" or change personalities. This idea is uneducated popular ignorance and It is really distructive for parents who need information from this century such that they are not scared to death to try good, standard medical care for a medical condition.
Second addition: Ther is a test for neurotransmitters in the brain. Unfortunately, it is done at autopsy. The same is true for older people who have alsheimers desease, but no one contests the existance of that disorder, nor blames parents, children, caregivers or victims of this brain disorder, nor suggests that medical intervention is inapproriate. Brains are flesh and blood people. What is off limits about medical intervention about any organ system with a defect? I just don't get it. If it does not work for your child after you have tried standard, good quality, medical intervention with a thoughtful and knowledgable professional who really understands these medical interventions after a reasonable and cooperative trial, then bash it. If not, then you really have zero idea how helpful these interventions are for children who really need them.