Ritalin sounds suspect to me, too - the other thing I'd consider is self-esteem issues and anxiety. My DS is in 2nd grade, and has the exact same challenges as your DS, with the inclusion of what we suspect is dyslexia. (His SPED team says they can't diagnose this - working on it now.)
I've noticed that with kids who are so intelligent, they are keenly aware of others' perceptions of them. Sometimes a diagnosis and meds can send them the message that people think there is something "wrong" with them, they are a "failure" and "incapable", and they need to take drugs to correct this "problem". This can be a crushing blow to their self-esteem, which is already taxed to the point of anxiety due to their struggles with attention that keep them behind peers in school (and sometimes the brunt of jokes from mean kids). It could be the meds were the "last straw", and while they help symptomatically, they are having an adverse effect emotionally. This was the very reason we decided not to put DS on meds, and to try natural remedies first...he is SO prone to self-esteem and anxiety issues.
Try talking to your DS, and see if something like this is underlying his sudden "inability" to do things. Intelligent kids will often hide their depression and anxiety from parents, and it will manifest in changes in their behavior.
Also, check out these articles I recently stumbled upon - they were incredibly informative for me in deciding how to manage our DS's issues:
After reading that first article, I started him on daily multi-vitamins with lots of B6 and iron, omega-3, phosphatidylserine, magnesium, zinc, vitamin D3, calcium, and probiotics. I give the calcium & D3 after school, and the rest with breakfast (calcium inhibits Mg & iron absorption). We already do gluten-free, dairy-free, soy-free, and follow the Feingold diet with no artificial colors, flavorings or preservatives, and low salicylates. So far, it seems to be helping a little, though it will take a few more weeks of supplements to know for sure.
Good luck - keep us posted!!