J.,
I would really suggest that you read about the biological nature of ADHD. It is a chronic illness, and you should really know all the facts. It kind of sounds like someone else in your family may have this same illness, and it is highly hertiable.
the gist of it is, your son has a malfunction of how one of his biolgical organs works, that it is his brain is not going to help him be able to make it work any better without intervention. The brain is made up of hundreds of millions of cells that do not actually touch, they depend on neuro transmitters (electrtically charged chemicals) to send information over a tiny space, called a synapse. If the mechinism in his body that produces neuro transmitters is defective, or if the receptors on the neruons do not uptake the neuro transmitter effectivly, then the person has ADHD and cannot always continue a thought process long enough to be conistantly to remember what it was they were doing. This can include all kinds of behavioral and educational thought processes.
Because he can do things well sometimes, you probably think that he should always be able to control it, and he is just being lazy or is deliberatly acting out. He is neither. Brain disorders are the only phyiscial illnesses where we accuse the victim of being in control of thier own symptoms, and it is very unfair and can actually be one reason that kids like this start to act out and bully.
Put any other symptom of an illness into the same situation and it just sounds mean. Take a child with a kidney ailment that causes incontientce. If he sometimes can hold his urine, but then later in the same day he cannot, you would not dream of accusing him of wetting himself deliberatly. ADHD is no less of a biolgical process gone wrong than kidney desease. If he was just going to be able to fix this medical issue by himself, he never would have had it to begin with. No child wants to be this way, and they have no clue how to stop it!
That being said, you are parenting him the way you parent him because typical dicipline does not work. You would not potty train a child with a kidney problem the same way you would a child with healthy kidneys either, so behavior is linked to most chronic medical conditions, in the brain or in the rest of the body.
Research your son's chronic medical condition and the medical intervention that is out there for him, it works well when you team it with talk therapy and family therapy so that you can find the kind of dicipline that will work for your family. All the potty training in the world will not help a child with kindney disease without treating the underlying medical condition...the same is true for your son.
Good luck, don't beat yourself up about your parenting skills. He has a medical condition and he needs help.
M.