I don't know what type he is or what medication he's on, but soju/beer is high in carbohydrates and will make his sugar spike. If he really can't leave the bottle alone he should look into alternatives... https://www.dietdoctor.com/low-carb/keto/alcohol-guide might help. Just keep in mind that when a liver is filtering alcohol, it cannot dump glucose. That means he could, if he's on blood sugar lowering medication or insulin, hypo if he's hammered, with possibly deadly outcome.
Is it possible your uncle is drinking because he is depressed about his condition? People respond to certain things in counter-productive ways sometimes... I do understand his reaction, but he's not doing his condition nor his liver any favours. Best would be to quit the drink altogether, but I do grasp that might be difficult for him.
If he's a type 2, he could control the diabetes through diet. https://www.diabetes.co.uk/forum/blog-entry/the-nutritional-thingy.2330/ might be of assistance... But again, I don't know his type nor his medication, so... But maybe it is enough to give him some hope, enough to turn away from the soju.