ETA again - An eval through the school is not going to be good enough. You really need to get an eval done by a pediatric psychologist. The evals the schools do are fine for most kids, but they do not diagnose. You need to know if your son has a diagnosis. If he has those types of sensory issues and meltdowns, he may have a diagnosis that needs to be identified.
ETA - I was responding to the sensory issues (for the most part) and the anxiety and meltdowns. He might be perfectly within the normal range. One of my sons is not, so I was thinking along those lines. Personaly, I would still request an evaluation so that you know for sure your concerns are being heard. I would keep notes for a few weeks on everything you observe and bring those with you so that you know you are being heard, taken seriously and can be given an opinion based on what you truly are dealing with.
Original Answer:
Academically, it can wait. It is absolutely ok for hiim to not have all of his letters or not be able to write his name. Those are things that many preschoolers pick up on, but many do not. I volunteered in both my son's kindergarten classes, and there were many, many kids who did not know their letters or shapes or whatever. Kindergarten is a strange time, because kids come in at so many different levels. I don't know how the teachers do it, but they somehow get all those kids ready for first grade.
There seems to be a big push these days for "kindergarten readiness," but academics is a small part of that.
I would talk to your pediatrician and get a referral to a psychologist to discuss the sensory issues and your other concerns. My son has many of those same challenges, and they are very, very real and need to be taken seriously. They can be debilitating to the child, but there are ways to deal with it. First you have to get the right diagnosis (so that your insurance will pay for things) and discuss the type of therapy or accommodations your son would benefit from.
I completely understand the fear of your child being "labeled." That thought makes me so angry! (Not angry at you ... angry at anyone who would do that!) I'm going to quote Dr. Suess:
"Be who you are and say what you feel. Because those who mind don't matter, and those who matter don't mind."
If you are true to your son and seeking help for your son, one of the best things you can do for yourself and for your son is to block out the negativity from others. There will be that day when you are at the grocery store and your son has a meltdown and people will stare at you or tell you that he needs to be spanked or ask if they can help (which they can't if he's having a;meltdown) or whatever. And it won't be easy. But you have to ignore all of that and just focus on what your son needs. And that will get easier. And soon (probably sooner than you think) you will find support from people who really do get it. And they will become the people you turn to.
Schools do not "label" kids. Teachers do not "label" kids. Schools and teachers do not always make the right choices/decisions for the students, but that is because sometimes what you try doesn't work and you just have to go back and try again. Work with the school, work with the teachers, stay positive, keep trying ... they want to help all of their students. In fact, they want to help all of their students have a "normal" a school experience as possible, and they will work with any students to help them have less time with the special ed teachers and more time in the regular classroom. That is the goal!
Talk to your doctor. Find the right specialist. Keep asking questions. You can do this!!!