I am a recovering person, so I have an understanding off addiction from two different perspectives-- as addict and as doctor. But I have not personally dealt with diabetes.
Having diabetes can be a very difficult, as you know. Young people with the condition logically have anger inside, and anger and resentments can fuel alcohol use. Alcohol also becomes a way to block the fear and other feelings that are part of having a chronic illness.
Your bf must realize, though, that he has a dangerous illness. I realize that he doesn’t ‘deserve’ to have diabetes; and it is natural to be angry and to have fears. But the alcohol WILL make the diabetes worse—in many ways.
People often stop drinking or drugs because of bad consequences. You, Elsa, might want to look at your relationship and see if you are somehow helping him continue drinking. And at some point you might consider leaving; Most people become by babysitting a drunk every night. I could see how the diabetes might make you feel guilty, to go find your own happiness. But it just might be losing you that convinces him to become sober.