Ruby, I'm not sure if this will help you at all, but something has seemed to click for my stubborn father in the past couple of months...
My father is an obese diabetic who needed cataract surgery; he got denied the surgery due to his blood sugar being OVER 300 when he went to the hospital (Thank goodness they checked!). He was put on insulin after the incident.
His doctor sat him down and gave him a talking to and asked him to do a simple thing - re-evaluate his own notions on food. The doctor didn't tell him to "Eat this" and "Don't eat that" but rather he said something like "If you think something is good or bad for you, why don't you look it up?" He gave him some resources (names of websites) and encouraged him to make pro and con lists of food.
My father actually did this on his own without input from my mother, me or my sister. He called me a couple of days later as though he'd had food revelations ("I always thought melon was bad for diabetics!!!") and he seems to agree with using the glycemic index for food, which is a good start. He started to tentatively make his own meal plans and according to my mother he's lost 18 lbs. in two months. He's still obese, but it's a good start.
It may be that if you ask your dad to look it up himself, he's more likely to believe himself? I wish I knew exactly what the doctor said to him, and it's likely the medical scare/being put on insulin helped nudge my father as well. I feel for everyone's pain here in dealing with a stubborn relative you're concerned about.
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/30/13 = 179.8 (net: -35.4)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."