Seriously there are two things you could be describing, and neither are withdrawal.
First - possibly rebound. But if it is rebound, even with long-acting medications like Concerta, you get the reaction in the afternoon/evening of the SAME DAY, with some run-over, less severe, to the next day. That is it. Rebound is like getting all the build-up of DHD behaviour that the medications have kept at bay, all hitting the child at once. Often also they can be teary, emotional, sometimes aggressive and angry. But it is most severe as the medications wear off then it eases.
Second - if the problem is still continuing, I think what you are seeing, is your child's normal state. Possibly aggravated by the fact that he's had medications to help him cope for however long you had him on the Concerta, and now he doesn't have the Concerta to help him. He will get used to having to function without the medications, but for ADHD kids it is a lot more difficult and they need a lot more support to cope.
Whether homeopathic medications or conventional medications, there is no magic bullet to help these kids. It has been described as a switch in the brain, in the inhibitory area, which works OK in us but not in these kids. The stimulant medications in these kids activates the inhibitory switch, making it more possible for these kids to control impulsivity and distraction. These stimulants for these kids are like insulin for a diabetic. And like diabetics, there is no effective homeopathic treatment for insulin-dependent diabetes.
Why did you stop the Concerta? Was there a problem with it? Was it too difficult to acquire? Because if he needs it if he is better on it, then there is no reason not to give it to him. However, if for hatever reason he cannot take it, he will eventually adapt to not having it on board. But his environment will be a much bigger problem for him and adaptations will have to be made, in order for him to be able to have a chance at learning.
There re people around who have ADHD but did not have medications. They get by now, as adults, because they learned to adapt. Did they do as well as they could have on medications? I can't say. Having seen the benefit of medications in my kids, I would say that doing without medications is not idea, unless you can really make a great deal of accommodations in their environment, and even then, only if the ADHD is fairly mild. The adults I know with ADHD who were never medicated often self-medicate (caffeine, cola) and if you are refusing medications, you also need to be aware of environmental stimulants such as caffeine and guarana.
If your child also has Sensory Integration Disorder (SID), I would be checking out Pervasive Developmental Disorder (PDD) in some form.
In our family, ADHD is a problem to a varying degree. easy child 2/difficult child 2 would have done badly at school without medications. Her brothers would have been completely unsuccessful. As it was, it has been a near thing for them and a lot of supports had to be put in place as well as medications. I could kiss the feet of the person who developed stimulant medications. We copped a lot of flak when we put our youngest on stimulant medications at the age of 3, but felt immediately vindicated when he at last began to talk. He was 3 years old, nearly 4, and non-verbal with jargon speech and a few single words. His older sister was away at a week's camp and noticed when she returned that he was speaking in complete sentences for the first time.
So if your child is bouncing off the walls more than two days after his last Concerta, then either he's getting caffeine from somewhere, or what you see is how he is when unmedicated. It will not pass.
Marg