What is the doseage for predisone for dogs/help with flea allergies?

My dog is a Rottweiler/Mastiff/St. Bernard mix. He is 8 years old. He weighs 100lbs. My dog has a severe flea allergy. He has been on a grain free diet, had a cortisone shot, been on flea medication regularly and still he gets really bad hot spots that puss. They come and go. Some days he will be worse and some days he is better. I am going to switch him from his current flea medication to promeris to see if this will help but my vet suggested getting him on prednisone. I was curious as to what the recommended dosage was for my dogs size. I have read the side effects of prednisone and am worried about that. Can anyone help me? Or have advice on other methods to try? I think I need to try the prednisone just to get it under control but its definitely not something I want to have him on for a long period of time. Thanks a lot. And please no rude comments or remarks. Just wanting information that is all.