I think teachers are overquick to suggest ADD these days. My own son's teacher made the same suggestion to me last fall. I am not a doctor, but I would think if your son can focus on subjects that interest him then he definitely doesn't have ADD.
Lets face it, most boys are physical creatures, who would rather be outside running, than sitting still doing math in class. When you look at the numbers, the majority of children diagnosed with ADD are boys.
That's not to say I don't think ADD exists, I just think that it's easier for the teacher to have all of her children quietly listening in class, and ADD drugs help her with that.
My own son is also 8, and can't sit still if his life depended on it. But put him in front of something he's interested in, and he'll stick with it all day. There's nothing wrong with his attention span - for example - last night he spent 3 hours intensely focused on the computer while he played a math game. He was just bored or struggling with the subjects being taught in class. He's really good at math for example, and would be bored during math time because it was too easy. But he struggles at reading, and would fidget. Maybe something similar is going on with your son.
Best of luck.