My dad was diagnosed with diabetes but he seems to be ingnoring it. What shall I do?

My dad is 51 and was recently diagnosed with Type 2 diabetes about a month ago. He still eats lots of lollies, soft drinks, and desserts. My mother has tried to talk him out of it a lot of times but he won’t listen to her, or to anyone else. He doesn’t exercise but is not overweight. He doesn’t seem to care much about his health and din’t even tell us until a week ago. When should I start worrying, what shall I do, and what might happen to him?