Hi Kristina,
Your grand father has about the same age as me, so I know how he feels...
Have a look at this:
http://neuromuscular.wustl.edu/mtime/mgrx.html
You will have a complete view of the range of possible treatments.
In your list, I recognize Mestinon, 60 mg 4 times a day, Prednisone 80 mg and Imuran which are quite adequate for MG.
You should be cautious with his food which should be minced and mixed so he could swallow it without chewing. Small meals but several times a day, he should drink liquids with a straw by small sips to avoid false routes.
Prednisone should be taken with his breakfast and the normal end dosage is 1mg/kg but it is normally increased day by day to reach the full level, if possible. Mestinon should not be taken on an empty stomach, always take something with it, milk, juice, chocolate...
Keep us inform of his situation and if you have any question, do not hesitate!
Maurice.