If the double vision resolved with prisms, it is due to a muscle imbalance and not the lens in the eye. Most likely source of all his problems is the diabetic damage to the back of the eyes. Can't comment specifically though as I have not examined his eyes. Your ophthalmologist should be able to explain the reason for the blurry vision.
HV
Just a guess, but I have had cat. SX in only one eye and now have a small amt. of double vision. Mine is because my IOL lens in my cataract eye put me into the myopic range -.60 ish and my other eye is in the hyperopic range +1...hence the two eyes have a hard time blending the 2 images.
After several months, it got better and tolerable. The brain is amazing and can adapt if the diopters are less that 3 diopters apart (usually).
Ask more questions...find out what your individual eye diopters are and see if that could be the issue. If so, perhaps give it time so your brain can do its thing by neuro-adaptation.
Good luck, ask more specific questions and do some more reading.