Tell him your concerns about him being overweight and try to get him exercise with you. Dont push him too hard to do it and ruin your relationship with your dad, because you never know when you can lose him. My dad just passed away last month, and he weighed about 300 pounds. He was 52. We tried to get him to take care of himself, but he had to want to do it and never did.
Do you and father both a favor and just talk to him. Tell him how you feel and if HE wants to exercise he will. Dont ruin your relationship with him over this.