Our child was diagnosed with severe anxiety at six years of age. She was placed on Prozac (which is one of the SSRI's as is Zoloft). After a very short while, she become another child - out-of-control with nary a care to what was right and proper. I believe this was due to the dosage being increased too much too fast. I really believe this is what you are seeing with your son - our child psychiatrist now says "go low and go slow". At the time we thought that she was "allegeric" to Prozac but, in retrspect; I believe it was the dosage amount and speed of delivery. She now takes another medication.
Our child was also suspected of being ADD/ADHD but as anxiety can mimic the behaviours of this disorder, this proved not to be the case. Anxiety also inhibits learning and this could be the reason why your son is unable to learn at school. I bet he is able to learn in what he perceives to be a safe environment (as his home). I would be really careful about the medication issue - as Dr. Kennedy said - be sure to get the correct diagnosis from a medical mental health specialist as a child psychiatrist before you tread much further down this path. In addition, anxiety is often co-morbid with sensory issues (sensory integration disorder) and I suspect that is the reference to which you made in your first sentence.
Also, I would like to comment on your statement about giving your son a "non-stress filled life". If anxiety is the issue, then know that this disorder is genetic and usually is not a result of life experiences. This is not your fault - neither is the colour of your son's eyes - it is what it is. Nonetheless, if anxiety is the issue, this disorder is quite common and highly treatable. Today, our child is doing fine - someday, I suspect, your son will be too. All the best ...
Your son should be evaluated by a child psychiatrist. It is important, as you know, to establish an accurate diagnosis and that has not yet occurred. He is young to settle on an ADHD diagnosis, though it is not unreasonable to do so. He may well display an anxiety disorder, but that should be determined by a child psychiatrist.