My father's Cretanine level has risen from 4.1mg/dl to 4.9 mg/dl within last six month time frame. During this period he is on medication to control the Cretanine level in addition to his regular diabetes doses and blood pressure drugs.
In last six month period he has restricted his diet, stopped eating non veg., is taking very less salt in the food and reduced protein in take as advised by the doctor. But when the test results came the Cretanine level has been elevated to 4.9mg/dl. My questions are as follows:
a)     Is there a medication to reduce the Cretanine level to the normal range which is about 1.5mg/dl?
b)     Due to old age (Age 64) and diabetes it's really hard to go back to normal human Cretanine level range but is it still possible to maintain the current level of the Cretanine (4.9mg/dl) with the help of the drugs?

Please advice.

Brief history: His diabetes is around 28 yrs old. From last 6-7 years he is using Insulin Injections before that he was on Diabetic drugs.

Optional Information:
Male , Age: 64