Most patients (including me) want to sleep the day away during radiation treatments and for weeks afterwards. You body has been through quite as bit and the need to shut down and repair is real. It also can be a symptom of someone who is clinically depresssed, and the lack of desire to get out and engage the world can be very real. It is easier to stay in bed - isolated and sleeping. I would confirm his mental state.

Confirm that his thyroid functions are OK. He could be hypotyhroid from the radiation treatments (especially XRT) and this would be a primary symptomology of it. Simple (often lifetime long) synthroid pills reverse the situation quite rapidly. Personally I think this is unwise and have never heard of anyone being on Ritalin for its energy properties (it has the opposite affect in adults from that that it has in children) for which it is an often adult abused drug.