Your poor little guy, he's probably a little frustrated with himself too. If he can get into TV or a video game, he probably isn't ADD/ADHD in the classic sense. The kids I teach that are truly ADD/ADHD, can't even do TV or video games for longer than five or ten minutes unless they are medicated. (I can tell the difference between the real ones and the ones whose parents are just fed up because their child is high energy. And there is a dramatic difference.)
Here are my opinions/suggestions. First, he is only eight. For some little ones, it takes years to learn how to focus for longer than five or ten minutes a shot. (Some experts suggest that TV and video games delay or prevent the development of this skill, I'm more inclined to agree than disagree, but I haven't read any studies myself).
Second, he is on a number of medications. Investigating all the potential side effects-and at this particular age- may shed some light on some of his troubles. If you find this may be the cause, perhaps changing the time medicines are taken can help with the sleep troubles, or tiredness, or lack of focus. Your doctor might suggest reducing dosages, or changing the schedule. Talk to her. (Actually, if I were the parent, I would do this first.)
Third, absent any clear cause of a "stomach ache" (like eating the wrong thing, the flu, motion sickness, the meds) they are usually a symptom of stress in children. If they just started or became more frequent this school year you can reasonably link it to something with his new class this year. It doesn't mean something wrong is happening, it might just mean that he is having trouble adjusting. If this is the case, listen to his tales of his days, he likely won't know what it is, but he'll be giving you clues as he talks to you. It could be something so simple as the teacher's rule about getting a drink, or using the restroom, or something more serious, like a kid teasing him.
Fourth, I have never heard of any child that actually responded to a diet change. This does not mean it doesn't work. As teachers, we are taught NOT to make any dietary suggestions to parents concerning their children (a big no-no). We all know the sweets and sodas make the kids bounce off the walls (no duh). Some schools have rules about goodies kids can bring to school. There are all these suggested diets for kids with different "problems."
I had a cousin that had to have his diet monitored closely or he would turn into a whackadoo. It would happen pretty quickly after he ate stuff he wasn't supposed to. It was easy to figure out what the food items were. Unfortunately part of being a kid is eating all that junk. You can watch it when he is around you, but you can't control what his friends might share with him when you aren't around. I understand that for the real effects to start to show, the diet/nutrition thing is a long term proposition. (One of my classmates in law school is ADD/ADHD and he had to give up refined sugar completely-months before classes started-to get his body chemistry ready. He's also on meds. He said they don't work if he has sugar in his diet.)
Fifth, since he seems to be holding it together academically, hold off on any special services testing until you can rule out side effects from the medicines, and see if he can get a bit more sleep (lack of sleep is actually a really big deal, it reduces immunity, energy, ability to focus, perform, retain, and just generally cope).
He sounds like a sensitive boy, if something is troubling him it could be causing nearly all these symptoms you described. You will figure this out. You're a good mommy.
Take care, Shoshana