P.M.
First, "I am getting to a point that I am not sure what to do any more" - If you haven't already done so, you may want to contact your pediatrician and advise him/her of your current feelings about the situation. They should be helping you & supporting you as well as your son.
You mention you have had him "tested by many". Do you recall whether or not he was evaluated by a Pediatric Occupational Therapist? I ask because ADD/ADHD are very often misdiagnosed sensory issues. Only an OT can properly evaluate. That OT should specialize in pediatric OT and in Sensory Processing Disorders.
At 2 1/2 yrs, my son was diagnosed w/ SPD. Because he was my first child, I thought many of his behaviors were typical for boys or typical for toddlers. Even though, I'd spend many nights crying, wondering why we had such difficulties accomplishing simple & routine tasks; why we were lacking joy & fun. I would jokingly refer to him as my manic-depressive baby. One minute we'd be laughing, playing, and having fun and in the next second (literally) he'd be angry, frustrated, screeching or crying and I just couldn't figure out what was setting him off. He would hit, bite, kick, spit, push, or throw things when he was having difficulties.
Having him evaluated and properly diagnosed & treated has been such a sanity saver. Seriously, in his first session, I cried because I had not seen him so relaxed in such a long time. Because I was dealing with it all day, every day I couldn't see how unhappy he had become, until he was able to relax w/ OT. And he loved it! No meds, just play (therapy, but play). It gave us immediate benefits which only improved over time. He's a much happier child and I'm much less stressed. Sure, we still have difficult times, but with knowledge comes power. I have a better understanding of what sets him off. I now know some ways to prevent those situations. And I'm more patient with him knowing he doesn't always experience the world as I do and have control of his neurological responses. As he gets older, he's starting to recognize when he's feeling out of sorts and sometimes makes the suggestions on how to correct himself. Mostly, we do our "homework" by offering him OT approved, sensory exercises daily to help regulate his nervous system.
Another excellent resource is the book, "Is this your child?". Many behavioral problems are caused by, or at least exacerbated by, undiagnosed allergies or food & environmental sensitivities. Through this process, I learned that my daughter's eczema always became worse if she consumed milk. I have since switched her to soy. I have also learned that my son is sensitive to citrus. Based on diaper rashes and extreme mood swings and hyperactivity, he cannot eat pineapples. He can have oranges but only small amounts once a month or so. Also, I switched him to organic milk. He started sleeping through the night most nights (at 3 1/2 yrs old, this is long overdue). He no longer screeches and his mood swings are quite so extreme. It's mind boggling how things we take for granted can have such a significant impact on our children.
Other things your pediatrician may suggest is a possible need to visit w/ a behaviorist or psychologist. Is there more going on than the ADHD?
Best of luck in your search,
P.