You son may be suffering from a lack of self-identity (common for middle children) mixed with the usual late tween/early teen angst (who am I? what am I? feelings which are common in this age), and he may then feel like he is worthless (or, at least, unsure of his worth) to the family and to his school community, so therefore, he feels no need to apply himself. I am not a fan of what I call the ADHD revolution. I can bet you that if you test him for ADHD, he will likely test positive for it, even though he may not actually have it. I think teachers and doctors are often overly sensitive on this issue, in part because the fix for it is so easy. And if anyone takes ADHD medication, they will be better at everything they do because the medication does help you concentrate better. So anyone will "improve" with ADHD meds, but it often does not solve any underlying problems. I would not rush to test your child for anything yet. Rather, I would try to tie him into something from which he can draw confidence and acquire skills with time. For example, learning an instrument, getting into competitive sports, or cultivating some other skill you know he has (art, music or singing, writing, chess, whatever). I would also ensure that his environment at home is structured, and the same for his 12 year old sibling, who in the same late tween/early teen category as himself. IMO, that means 10 hours sleep nightly, limit TV and video games or eliminate them altogether on school nights, dinnertime together as a family, and strict rules or hanging out and communicating with friends, with consistent consequences.
By the way, I hope not to offend anyone on my opinions regarding ADHD. I have seen real ADHD and it can absolutely be a debilitating disease. However, since I am not a fan of placing children on long term medication, I am recommending other ideas first before getting involved in the LD/ADHD search, which I often see from other parents as a long and sometimes frustrating process in search for answers.