I am in a very similar boat with my father. I didn't see any mention of whether or not your dad has a wife/partner, because if he does and she can be of any help, my first thought is to partner up with her and see if you two can help coax your dad in the right direction.
My dad is probably up near 280 or 300lb and is a big guy (barrel-chested, sturdy build), but he is definitely obese. I'm sure that that's where I've gotten most of my (old) poor eating habits from. My dad is extremely intelligent, logical, and ridiculously bullheaded. He refuses to listen to "his kid" about nutrition, because he's the king of his castle and how could I know something he doesn't. Usually he's right, but in this case he's wrong.
I don't have much sound advice besides "you're not alone". A lot of guys from the older generation are SO set in their ways, even if their ways are detrimental to their health. They have doctors and nutritionists telling them what to do and how to change their ways, and it's just waved off with "oh, I'm not doing that, it's stupid." My dad had a heart attack 4 years ago, and he has cut back on his sodium and red meat (to a point), but he still sits his arse on the couch with the "low calorie" chips (and eats the whole bag) and routinely brings cakes and breads and meats into the house while I'm trying to make quinoa with grilled veggies and lean chicken (which is delicious, by the way).
He gives me this line of nonsense about his "quality of life" as though it's an excuse to eat poorly and be lazy. My mother knows he's doing himself wrong but there's only so far she can push him before he does the "woman, please" dismissal. He considers a slow meandering walk with the dog around the block to be his "cardio activity", and then he has several drinks and polishes off some kind of white flour, processed sugar treat as his "reward".
Frankly, I have almost resigned myself to the fact that he is probably going to die "young(ish)". I'm not going to suggest that you threaten him with ultimatums, but perhaps you could discuss with him that you won't be physically able to care for him if it becomes necessary. If the funds for personal care aren't there either, he may be in a really tight spot if he continues on his current course. If you've already had this talk, it might be pointless, because I'm responded to with an eyeroll and a "yes, dear, thank you for your concern" dismissal.
I'm not much help, but believe me I know how you feel. Best of luck, and if I figure out anything worthwhile I'll get back to you!
2009 -- 280 lb+
3/2012 -- 190 lb
4/2012 -- fell off the wagon with a BANG
5/2012 -- 218 lb
Next goal: 200 lb by July 31