How can a stubborn 48-year-old lose 80 pounds?

Let me explain the situation: my dad is 48 and he's 212 pounds. He still thinks he's healthy (weird isn't it?) and he's a very stubborn guy. He practices Tai Chi, and he's a businessman and has very little free time. When he does have free time, it's spent watching television. When he was young, he used to be very skinny like 110 pounds. But around his thirties he started getting more obese until he reached today, around 212 pounds. Not to say that Tai Chi is bad or anything, but he rarely practices it and it's not really working. How can I get him to lose weight and how long will it take for him to lose 80 pounds if I get him to exercise?