Typically, I would ask several more questions about symptoms, etc. But, in this case, it does seem kind of obvious. Heredity, is an important clue. His teachers' comments also are important. But, to me, his coach's comments are perhaps the most important. I am assuming that he is involved in a sport that he likes. And, if you really, really like a sport/subject and can't stay focused that is a very important clue. It takes it out of the typical teenage boy thing.
I wonder if the coach sees any difference in him at the end of practice then at the start. Of course, I don't know what the sport is - but, lots of studies, have shown that intense physical exercise increase dopamine and that effect can last for up to several hours. If he is in something like soccer, I would expect to see a change at the end of practice. If he is on a Chess scholarship ... not so much :)
It does sound like he is a very intelligent kid who academically has been able to get by on native intelligence. I have had many posts with kids/students in college or even graduate school who were able to get by - until, it just got too difficult. At the middle school level, it was typically something like algebra that caused them to hit the wall.
So yes, I would get him tested. And, actually, you might see if the school would do it as it would help with 504's etc. But, at this time in the year, they may not be able to get to it. So getting it done your self will expedite things. And if you choose to use medication, you will have the time to get the right dose before school starts.
Let me know if you need any resources for him ... or any of your family. Best wishes.
Here is a good link on ADD which you might find helpful.... https://www.additudemag.com/slideshows/symptoms-of-inattentive-adhd/
Yes, ADHD without Hyperactivity seems really likely in that case. That form of ADHD also happens to be the form I have. I was the same in that I always excelled in school and seemed to concentrate just fine. I've heard the problem in this form is sometimes not a lack of attention but a lack of the ability to concentrate on the right thing. It became a significant problem for me around that age too. Puberty tends to make disorders worse. You should probably try to get him a diagnosis. It's really frustrating when you just can't get your mind to stop drifting away from the topic at hand! The thing that really sells it for me is that your son seems to be missing instructions and sequences of requests. I always need to have explicitly written directions because even just a few basic requests given verbally immediately leave my mind and I forget every step after the first one. And sometimes including the first one.
As Sandman2 says, there is a genetic component that makes it more likely for your kids to have it if their siblings do. He really could use a 504! Good job catching this early, by the way!!! It took YEARS for my family and I to figure out why I went from "Gifted and Talented" to failing classes in a matter of 2 years.
I was diagnosed with ADD (without hyperactivity) around age 12 and had been really good at hiding my inattention until that point. It is lucky I got tested to find out what was going on! There's no reason to fret-- he will likely be able to do even better than he's doing now after some strategizing for his particular learning style. It's always a good idea to test. I ended up being diagnosed for visual perception issues and finding out I was an auditory learner, which I never would have known had I not been tested! It helped me seek out subjects and study methods that would work for my brain.