Alexandru, I do understand the problem, as I have most days (and sometimes nights) a very unpleasant period when I cannnot move at all. At such a time any strength I still had, has left my muscles, any control over my limbs has almost completely gone (the arms are slightly more willing) and no matter how hard you try you cannot overcome this. Your muscles tend to seize up and you feel very uncomfortable. This is what PD is about, it is after all frequently called a movement disorder.movement Only drugs can help to get you out of this situation (I got stuck in the High Street and struggled leaning on others to get to a chair in a shop.. That is where the Apo-pen can be useful as it is faster acting than Sinemet or Madopar or Stalevo..
Exercise....I have a problem with the claim of it being of any help in a situation like that. In the early stages, yes, but I think that eventually in the course of the disease that does not play a role anymore, you are in a more advanced stage and the drugs are starting to become unreliable in their action. This is the time where the "On/Off" phenomena start to disrupt your plans for the day. You might not make it to the toilet, you are not ready to go out, etc.
It is , however, important that your grandfather keeps moving as much as he can. Give him the time to go (no matter how slow) from one room to another. Do not let him sit still for long periods at a time (very tempting for a patient to indulge in this, but), movement will become increasingly more difficult . Encourage him and realise that he is uncomfortable , maybe in pain when he struggles to put one foot in front of the other. .