My ds was diagnosed 7 years ago, so I will share some of the information that I have learned over the years. It would appear that AD/HD is due to the right brain being more advanced that the left. The right brain is the "doing" and the "creative" brain. This is why many ADHD people have musical talents and draw well etc. Of course, not every person does as you have to have the talent there already iyswim.
Science is a doing right brain subject, so there again is way there could be an interest there. Maths, depending on what the subject matter is, uses both. Story sums for example, uses the logical left brain, so many ADHD children struggle with, but general arithmetic is quite easy for them usually.
Organisation - well that is definitely a left brain feature. They are not able to organize themselves well. Ds doesn't now at age 15 and hasn't ever. Little things like getting ready for school in the morning, I have always found a nightmare - although he tell him to arrange everything the night before, his organization is thinking where he left it last and will sort it out when the time comes. Tips for these: being aware that he isn't doing it on purpose to annoy you - he probably started to write down all the work in his homework book, then looked out the window, watched the person go past - wondered where they were going and what was in the bag he was carrying - then the class moved on and he didn't get to write the rest down. Just a little example.
Generally ADHD children score very high on IQ tests, just out of interest has he ever been tested? They will focus intently on things they enjoy, but faced with something they aren't too keen it, can make things difficult. When you write "averagely academic" i would bet my bottom dollar that he is actually above average, but the subjects that he doesn't enjoy so much (in our case with ds is things like business studies, accounting etc) are bring his overall marks down. Ds excels in English, he got full marks for creative writing, but is battling Business so his average mark is just average due to the fluctuations.