I don't think that's the case, if he was diabetic and wasn't taking medication for it, he would be losing weight, but he is quite chunky atm.
Usually diabetics are fat fucks, i know that, but if you have untreated diabetes you start to lose weight and you don't even notice because the glucose isn't entering your cells, it's staying in the blood - which causes the blood to become as thick as maple syrup, which makes it hard for the organs to get oxygen, leading to kidney damage and whatnot.
Fat people just tend to get tested for high blood pressure and diabetes more often, which is probably why they don't lose weight: as soon as the doctors find out their blood sugar is high they put them on metformin.