My brother had this problem and was finally diagnosed with ADHD as an adult. Now he takes Ritalin or something like that every other day. He only takes it on the days that he needs to concentrate and focus on things, and off one day because it is hard for him to be creative on the drug (he has to be creative for his job). So what about testing for ADD?
Is it possible that you are overbearing and try to mother and boss him all the time? And he is "forgetting" to rebel against you passively? No offense to you, just a question?
Or how is he with his job? Some men have personalities where they are so intensely focused on one thing, and one thing only, that they are very unmotivated/unable to multi-task. So is he really good at his job but bad at all the other little outside details? This could just be a personality trait.
Personally, the more stress/emotion I have on my plate, the more forgetful I am and I have had husband and coworkers amazed that I have completely forgot entire important conversations with them.