Please add to your post and let us know why you "have to be at school with him" and precisely what that involves. Are you at school all day, every day, in every classroom he's in? Many schools simply could not permit that -- I mean they truly could not PERMIT it unless the child had a specific, written individual education plan (IEP, it's a school document that requires meetings with administration/teacher/counselors/doctors etc.) and that plan required a parent to be present. Does he have such a plan? Has the school system arranged that you are formally the one who is expected to be present all day or is this all an arrangement where you and the teacher together just decided this is a good idea?
I am asking all these questions seriously -- not to get at you. I question why any school would let a parent be with a child the entire day unless there were a written order from a doctor asking for that, or unless there were a formal IEP in place. Is there a reason the teachers cannot handle him? If he's not ready to be in school at all because he is so extremely disruptive -- you need to take him out of school to focus totally on altering his behavior, and school can wait another year, period. Why does the school let him stay if he's so disruptive he needs a parent in the classroom every day? The focus needs to shift to him, not to keeping him in school no matter what.
Is he seeing a specialist to find out for SURE if the other doctor's idea about "slight damage" to his brain is accurate? Your son should be tested to see if that's true -- has anyone ordered such tests or are you just told, "Oh, well, we'll see what happens"? Don't let doctors fob you off like that!
Is he seeing a counselor or therapist outside school? It sounds as if he needs behavioral counseling both to undo years of being treated leniently and to teach him the basic skills of how to entertain himself and how to control his emotions, since he's grown up so far being very used to having his emotions catered to. Again -- NOT dinging you and dad here; you were baffled and tried your best. But there is a lot to undo because the first five years are critical ones for teaching any child how to interact with others and with the world.
Please get much more assertive with doctors, now. Your son is five so you have a good shot at reversing his behaviors over time, but it has to start now.
You don't need "tips," you need a firm plan. Make a list and work it hard -- it will require you to be very aggressive about getting him a therapist/counselor to work on behaviors; you and dad also should be seeing a family counselor who can help you figure out the best ways to handle and discipline your son. You need to get back to the doctor who casually mentioned "brain damage" (!) and force a referral to a specialist. You need to meet with the teacher, school counselor and principal and talk about an IEP or whatever tool your school system uses - if your son needs a full-time aide with him in the classroom, and it's public school, you will need to document medically or psychologically why he needs an aide and then demand one. You also need to be open to the idea that maybe he needs not to be in school yet and needs to be spending that time in medical and psychological therapies instead.
You and dad must get tough and start saying to doctors, teachers, etc.: "This cannot continue so by the end of this day you will get us a referral to a specialist; by the end of this week you will get us an IEP (or whatever is needed)" and so on.
And most of all, YOU must stop going to school with your son and stop being present for him every second of every day. Where is dad? Why can't he handle your son at home, if you are having to be at school? Your husband must understand that his wife is going to have a breakdown if this continues. And you both must reach out immediately for help and demand it -- if you don't, others will just assume that your son is badly behaved and poorly disciplined and they won't offer to help you because they can't see he has medical issues. You will have to pursue this.
I feel terrible for you. Your son has legitimate issues but you're letting him rule your every waking second. You cannot be a good mom to him if you are burned out and if you resent him deeply because you have no time for yourself. Go in today (without your son present, any way you can manage it) and talk to the teacher and do not leave until a meeting is scheduled for you, teacher, principal and counselor, preferably tomorrow.
Why can't the school handle him without you present? What kind of issues or meltdowns does he exhibit if you are not there? Is he in any activities to help him burn off energy? Does he