how do i maintain a constant sugar level?

Hello again, we have made it to Greece and my father is doing as well as can be expected with his cancer condition,  and also ok with his diabetes.  However because of a steriod that he is on for the cancer it causes his blood sugar to be very unstable.  Some mornings like this morning it was only 92 but other times its as high as 190.  How can we keep a constant level, as we have notice a direct correlation between his tiredness level and his blood sugar.  Because of the brain tumor we need to keep him as active as possible but sometimes when his sugar is high he refuses to get out of bed.  Any suggestions on maintianing a constant level would be great and websites to research diabetic diets for grek cooking.  Also know that his in on Humulin 70/30, 20 cc's, twice a day or as needed.  meaning if his sugar is lower than 100 I dont adminster insulin, is this correct?  We dont have a diabetes dr her in greece as the oncologist believe his time left is limited and basically left his health up to us.  My uncle is a pathologist here in greece so he has been looking after him also but still we are all new to diabetes.  Thanks in advance to the info.