It has to be his choice. You can’t make him do IF. Or eat right.
You can tell him how you feel and encourage him.
If he’s game, then focus on the calories. Start at 2000 calories a day, or use a tdee calculator like https://tdeecalculator.net and aim for eating under maintenance/resting.
And his walking and biking is fine. Calories are more important than exercise.
And make sure his doctor is on board with any plan he comes up with.
Does he want to start? If not there’s nothing you can do, it’s his choice and his body. I appreciate your worry for his health but it’s really not your place.
If he is keen, the best thing to do is give him any information you have and support him in his choices. He probably won’t do it like you would/think he should but he will find his groove.
I will offer an answer, & I applaud you in trying to help but here’s my disclaimer: Yeh um, good luck. Parents don’t listen. My dad has been fat for 30 years. I started doing IF when he started just to support him. I started at a “healthy” bmi just shy of 25. I lost 22lbs. He lost maybe 20 of the 50 he needs to lose bc he’s half assed it & is happy with that- but not healthy & his back & joints are extremely painful due to his weight. Ok, whew, back to you: high healthy fats, high fiber, high protein. Salmon with broccoli & sweet potatoes. Lots of soups & stews (although hus doctor might want him to watch sodium intake), beef, chicken, salmon. Fruits in the first part of eating window but personally I avoid in 2nd half of eating window.
Re: your dad, Does he want help? If he wants to, then start with either A) 11:00AM -7:PM 16:8 hard fast @ 8 hours, For easing into it. Or, if you fear he won’t stick for it longer than a week, 2:00-6:00P, a 20:4 fasting window for faster results. Idk man, there are a lot of variables here. Prior physical health conditions? Depression, medications, his own interests in weight loss.
I guess anything at least 16:8, just skip that 14:10 mess, bc 1) it’s barely IF & 2, with a 10 hour window it is way too easy to overeat. 2) imo it’s not really even IF as a high carb consumption diet will not even enter a fasted state long enough to make a difference before he eats again next day.
Re: snacks, if he drinks carbonated drinksstop those. I hate fake sugar but that’s me. Idk what his flavor water has in it. You caught me on a bad day & I’m grumpy AF, so I’ll go out & say it, humans should be content with drinking water, plain unflavored clean water as their primary (70% of it) liquid intake. Sure, some unsweetened tea here, some coffee there, tolerate juice at times.
Rant & not directed personally at you or your dad: But what irks me is people being so dang picky & not being happy drinking water. If i hear “just water” or idl “plain water” one more time imma gonna lose my mind. Stop trying to sweeten everything. Let your tastebuds adjust to enjoy cool clean water. Not everything has to be a party for your senses. Use a sqeeze of lemon if you need training wheels.