My two kids both have ADHD - but present in different ways and have very different strengths and challenges.

I have one diagnosed, and two more that probably have it (I'm 90% sure on one of them, and biding time on the other). For me, the question is what's the benefit of the diagnosis? I got the one diagnosed because he needed some accommodations and medication. The other one I haven't bothered to get diagnosed. I'm pretty well versed on ADHD stuff, so I try to work with her on managing it. She does well in school, except for getting marked down for having a disorganized desk and for blurting out answers -- but I can live with that for now. If she gets to the point that she needs accommodations in school or where I think she needs the meds, I'll look into a diagnosis then.
So, OP, I would say just to wait and see --- little boys are often very active and don't have well developed frontal lobes. Oftentimes, they just need a little more time to catch up. If it starts impacting him negatively (low self esteem because kids don't want to play with him, etc.), then go and get the assessment.

Question -- how is attention deficit disorder diagnosed, anyway?

There does seem to be a genetic compenent, although the specifics aren't yet clear.

Both DS's have it but with different presentations. DH does as well.

You might look into the NIH ADHD research study.