In order to figure out how much function he will have later on, if any, they will have to reduce the sedation which is keeping him in coma (if he is still on it- and he may not be). Then they will have to see if he can be weaned off the ventilator that is supporting all his respiratory functions. They can get some idea of how well his brain may work from things like MRI and neurometric studies (EEG for example) but really it's mostly a matter of waiting it out and seeing what he does.
The former Israeli PM, Ariel Sharon, recently passed away after a prolonged period in a state of minimal awareness if any- I believe eight or ten years after a spontaneous brain hemorrhage. He was much older than Schumacher and the injury was of a different kind. But unfortunately it is possible, with supportive care, to live a long time after a major brain injury, but not wake up. It all remains to be seen.
FWIW, I don't think he is suffering pain. But everyone around him is.