Motivation has to come from within your dad. He has to have a reason to get into shape.
I am 60 years old. Here is my story:
2 years ago, I decided that I should quit smoking. I decided that when my last two cartons of menthol lights were gone, I would quit. As the packs began to dwindle, I smoked fewer and fewer cigarettes to save the remaining few.
Eventually, I was down to 5 cigarettes per day, then they were gone. I was on edge, but the first two months passed without giving in to temptation.
I allowed myself to eat or drink whatever I wanted during those two months as a reward for giving up the butts. I gained 20 lbs. during that period. I then decided that I needed to get some exercise, so I started walking for 30 minutes every morning and evening. After a few weeks of this, I decided to join a gym.
During my visit to the gym closest to my house, I ran into a guy I hadn’t seen since the last time I worked out 20 years ago. That was it, I had a gym and and gym buddy.
For the first two months, I worked out alone on the lever and pulley resistance machines. I did a four day split of upper body/lower body/upper body/lower body with rest days between workouts. This got me in shape to start training with free weights and join my buddy in his workouts. That was almost 2 years ago.
I am now in the best shape of my life. I workout 6 days per week and do cardio for an hour every day. My weight has gone from sloppy 196 lbs down to a trim 168 lbs. My waist has gone from 39" to 32". I am healthy and have no need for medications.
Your dad can do this, if he wants to.