Honestly, as someone who comes from a family with similar predicaments-- there isn't really a way that you can "get him back into shape." He has to want this for himself and he has to be willing to put in the effort every day to slowly change his lifestyle back into a healthy one.

Weight loss takes a lot of time and effort and doesn't give immediate gratification; his health and looks will take weeks, months, years to improve whereas drinking a Coca-Cola or binge-watching something on Netflix provides instant satisfaction. So while you can't push him to do something he wants to do, you CAN be his motivation and support during his weight-loss journey.

First you should really lay out things to him to make him realize the gravity of the situation. Don't shame him, but rather lay out the health consequences, some statistics, and maybe discuss personal issues he is having in relation to his weight. Try and have your mother (separately, not together, it's important these discussions occur privately so he doesn't feel embarrassed or attacked) bring up some issues she is having with it as well.

Make sure you reiterate it's about your concern with his health, mental and physical. Once you outlay that for him, it might help give him a bit of motivation. Another thing for you to consider is that the reasons diets tend to fail is because they're such massive lifestyle changes that are completely unreasonable. People who are used to spending most of their time at work and the rest of the time watching television or winding down from their day aren't going to want to change, go to a gym, work out, shower, and spend the rest of their free time cooking and sleeping. It's too much.

What you should instead do is focus on incorporating exercise into activities rather than making it an activity of itself. Encourage your family to go to a zoo, an aquarium, a hike, some outdoor adventures where you can have fun together while getting some moving done. Perhaps buy a treadmill or another similar machine with a television in front or radio around so that it won't be a boring exercise. Even swimming would do a lot of good.

Now, another important thing to consider again from what you've said, your father doesn't seem too lazy. He's been attempting dieting and at least is aware of the situation, so the talks and the slow incorporation of exercise will probably go over somewhat easier. What you need to realize is that these diets are designed to fail because they're simply unsustainable, and that weight often reflects food choice. So if you REALLY want to have an active role in health, educate yourself on some easy recipes and start cooking for the family.

Replace copious amounts of butter, salt, sugar, and alcohol with proteins, vegetables, fruits, and water. Try cooking things that taste good but are also healthier. Keep in mind if someone is eating McDonald's like every night then they're not going to immediately want to go to your veggieburger with fried kale. Instead, start cutting out fast food and cooking more at home. Try substitutions for food that are simple, such as almond milk or 2% instead of whole milk. Encourage him to have only one soda a day instead of two or three. Offer to go shopping and buy things like flavored water and juices. Buy sugarfree gum. Pick gluten-free burgers to cook with. Cut off more fat from the meat that you cook. Start baking things instead of frying them. Use less cheeses in what you cook. Venture out into trying different foods unfamiliar to you/your culture. Try incorporating more rice into your meals.

You can do your best by cooking, encouraging family exercising activities, and by showing him statistics/medical facts to try and have him realize his position. But past that, his motivation and determination for a change has to happen from within. And remember, diets are mostly designed to fail and are unsustainable in the long run-- you have to do those little changes slowly and over time instead of attempting to enact a huge change. There's a difference of being fit/in shape and average/healthy, take it one step at a time and focus on little goals rather than the overall plan-- a pound a week is a great thing to establish, rather than "100 lbs in two years."

Good luck and remember to support him even if he fails and caves in from time to time-- weight loss is a slow but constant battle and will take some time to fight.