DD has certain things that she DOES do very well in in terms of attention, for instance she read most of a Harry Potter book in one sitting. She can put together a large jigsaw puzzle. Even unmedicated. But the problem usually comes in if there are lots of distractions around her and it's not something that she enjoys a lot or is engrossed in. So writing and math are very hard for her when there are lots of distractions but she does Ok with reading. She uses headphones in the classroom for some work but not other types. She is very slow with written work when she's not medicated (slower than usual-she is always slow with written work). So even with processing speed there are differences. It affects her math and writing but not reading fluency. It doesn't sound like it's a clear cut obvious case of ADHD but I wouldn't rule it out just because there are some things where he has good attention. If he is very inattentive in certain circumstances (like things that are boring to him but he is expected to do) that would be a red flag. I kind of like the computerized test at the neuropsych that DS did because the kids know they need to do it and most of them will try, but it is super boring and it gives an objective measure of how impulsive they are in their answers and whether they start losing focus after a while.
