A couple of things.
First, the medications. If they're making a difference in a good way, you stick with them. If they're not, you don't. Temple Grandin calls it the Wow! factor - if you try something (medications, therapy, behavioural approach) and notice "Wow! What a fabulous improvement!" then of course you fight to keep it. But if it's more "meh - I spent money on THIS?" then there really is no point keeping on with it. So if that's the story with the Concerta for your son, then I don't think the doctor will fight you. Unless the doctor thinks the dose is not high enough. But even then, if it was going to help, there still should have been some improvement even on a lower dose. If you and the doctor can't agree then I think you'r making a wise decision to try someone else.
Now to immunisation - you mentioned your own history of what sounds to me to be an oversensitive immune system. Yes, it runs in families. The problem in these situations generally is not the preservative, but the immunisation itself - anything that stimulates an already oversensitive immune system can trigger unexpected effects which, once triggered, continue on their chaotic way. it is rare, but it happens.
I mentioned a case of a kid we know who had a seizure disorder at birth but doctors still immunised the baby. The fitting became a lot worse and now the kid has a diagnosis of autism. The mother is very anti immunisations and especially insistent that thiomersal (or thimerosal, whichever it is called in whatever country) is the cause. I had investigated this already in Australia by calling the authorities and found that in Australia, we have not had mercury additives in our vaccinations since long before our kids were born. But when I told her this, she dragged me into the library of the disabilities group and showed me a book which listed the vaccinations that still contain thiomersal. It contradicted everything I had independently found out. Then I checked the book - she had written it herself. But she was using a book she had written, to verify an argument she was making!
I don't know if additives were responsible for your son's problems or even if the immunisation was a cause or even a contributing factor. But if YOU have a sensitive immune system, be aware that the purpose of immunisation is to stimulate the individual's immune system to develop new antibodies in order to give protection for the individual, as if they had already had that disease. Perhaps because some immunisations are increasingly easy (such as the oral Sabin for polio) we tend to think about them as no big deal. But they do have an affect on the body. In most cases it is a beneficial effect. But not always.
I am pro-immunisation, but I myself have an oversensitive immune system and had one immunisation too many, with resultant catastrophic effects. In my case it was the immunisation itself, not any additive. It was my immune response (an atom bomb trying to crack a walnut) that set off damage to my nervous system. Later on I still had my kids immunised (with heart in my mouth!) and thankfully there were no problems. But I definitely do accept that sometimes problems happen, because every body is different. I am my own example.
One point I really want to emphasise - even if it was the immunisation that did this to your son, even if it was the thiomersal component and its mercury that did it (and there isn't any way to know) then the problem is most unlikely to be mercury now. There are fortunes being made by people selling chelation procedures (which are often more dangerous than the stuff they're trying to clean up). And if the damage has already been done by what would have been at the time a microscopic amount of mercury, the body will almost certainly have already dealt with it. I've used the example before - if you slip on an icy path and break your leg, clearing away the ice won't fix your leg. You have to let the leg heal anyway.
So if you are still concerned about the possibility of mercury or other heavy metals still being an issue, PLEASE go carefully! There are more sharks out there than genuine practitioners. The sharks are more dangerous in their treatments, they care a lot less and they are very, very persuasive. In fact, they have to rely so much on their persuasion that it can often be your warning - any doctor trying to use emotional blackmail on you ("don't you WANT your child to be well?") is likely to be shonky.
There are good doctors out there who will advise on this, but I personally feel, from what you say, that it is not currently an issue for your son (unless he spends his time playing in an area contaminated with heavy metals). I said I had researched this - I researched it here in Australia, so my advice is specific to here and not about what is in your market, in your area. But the same shonks are everywhere and they prey on people like you and me. been there done that!
I believe immunisations are, on the whole, a good thing. But sometimes they are the wrong thing for one particular person. And when that happens, we have to deal with the result. We can't go back and un-ring the bell.
Oh, and I have seriously thought about it with us - since in Australia the first immunisations are at 2 months old, how can I know that it wasn't immunisations that did this to my youngest? It's because when I look back, I can see the signs of autism in him from his first few days. We came home from the hospital at 6 days, and I found out VERY fast that he would settle like magic, when we were out among the trees, especially towards the end of the day. Or the beginning. He liked it when I put the tree between us and the sun (as you would, especially with an Aussie summer sun!). But he would turn and look at the tree - at the flicker of light in the trees. And he kept doing it, for years. probably still does. he has never flapped his hands - he never has needed to. The flicker of light in the trees has the same effect on his brain, it soothes. And he did it long before his first shots.
So even though he has inherited a wonky immune system like mine (he also has allergies), I do not believe, in his case, it is the cause of his autism.
That does not mean it is not an issue for others. But it is multifactorial, as you have said.
My worry for you and yours is similar to my worry for me and mine - we live in a world which is increasingly challenging for immune systems like ours, and increasingly, our bodies let us don in various ways. I don't know if your son's autism was caused by the shot or not. And really, you can't be sure either. BUT - we know we are vulnerable, you and I. And our kids. So we have to proceed carefully from here.
As parents we do the best we can do for our kids, and we can never get it perfect. We have to forgive ourselves when that happens. It's Life, really.
I'm glad you're on board with us. I just felt I needed to clarify what I said before, and give you a bit more info on where I personally am coming from. I do not require everybody else to agree with me. The world would be a dull place if that were so!
Marg