I have an idea. I think you should sit down and write your Dad a letter telling him how much you love him, and how worried you are about his health. Tell him all the things you want him to be around for in the future: your graduation (if you have not yet), your wedding someday (if you plan that). Use your own words, but you get the idea. You want him in your future. You need him to be part of you life.
Then if there is any kind of exercise he ever liked: going fishing, taking a hike, give him a coupon book for 15 hours (one hour each) of those activities with you. If he never liked any exercise much, ask him to please take a walk with you. Start slowly. Tell him you want to have some one-on-one time with him. (Write the coupons that way.)
The gift of yourself and knowing how you feel is the best gift he could ever have.
Remember one thing though. In case this does not work in the long term, we cannot, no matter how much we want to, change other people. We can only make our case, and then step back.
Good luck.