I would say, and am in NO way an expert, just a mother of a teenager, a preteen with ASD and possibly ADD, and I have ADD.
If you feel that he has generally been able to cope, and has managed a normal primary school workload, and has only now shown these signs, they absolutely would fit with anxiety, or a similar issue. My 14yr old undoubtedly has a slight ADD issue, but generally has managed without intervention (mainly because he’s much cleverer than me!), but last year he suddenly started exhibiting all the symptoms you list.
He finally cracked and showed his absolute distress because something awful was happening with a friend of his, and he had been sworn to secrecy.
Speaking to the mum of said child, he was also acting out in the same way, and not only not concentrating, but apparently no longer interested in any of the subjects that he had easily done in his sleep.
Bottom line, find a way in. For mine, I actually expressed my sadness that he was being such a tosser, and that he was making us all miserable. I’m not proud of it, but I suppose I was letting him know that I was not coping, could not find an adult way to manage his behaviour and that was scaring me. And it meant I was being really unpleasant back (shouting, mostly). It was late at night, he was exhausted, and he just burst into tears.
I’m not suggesting that is the way to do it, just that I recognise I”m asking a lot for you to figure it out. Maybe check in with his teacher? For some parents that will be met with a snort of derision, but for some ,they’ll have noted a change too, or noted a change in who his hanging out with etc etc.
Anyway, good luck. I”m pretty sure it’s not ADHD, but of course the caveat is that we only found out that DS2 was dealing with ASD because he became so anxious that he couldn’t manage school. He actually held it all together SO well that we could explain away many of his characteristics as “just quirky”. Now he’s relaxed and less anxious, he’s so much more Aspie.
Not sure if any of that helps, because Ive just offered two completely opposed outcomes!
Anyway, good luck.