You are right, your father is going to have to change. Some things I suggest is maybe your brother could come over a couple times a week and the two of them go on a 30 minute walk. You could see how your dad feels about a dog too, they need to be walked every day or they too will be come fat. Plus it will give him a companion. But it also brings more responsibility, so that has to be totally agreed upon.
My dad is 60, and he loves facebook, especially farmville. He could get back in touch with people from high school the same age (if he's not on facebook now). Your sister could talk more easier with your dad on facebook too, of course your brother and you could too.
I'm assuming you are planning on coming to see him at least once a week. You could go grocery shopping with him during your visits and make sure he does buy fresh fruit and veggies... Maybe make up some meals for him too. Either bring them with you or make them at his house. Plan an activity even if it's just a walk around the park. I know my dad loves playing with the grad kids, baseball, going on adventures in the woods, you name it... You don't have to spend money to have fun either.
Just talk to your dad and let him know how important he is to you. I how much you love him. He's your only parent alive and you want to enjoy every minute you can with him and you don't want to lose him either. Dad's do understand, see where he's at and what he wants. You know your dad, maybe your brother and him can go fishing if that's their thing.
That is so sweet how much you all care about each other! I have a family like that too. It's a true blessing!