At 2.5 years, your dog is the size he will be. He may bulk up just a bit more, but he wasn't meant to be a 80 lb dog. Find a good quality food, top it with the chicken and feed him on a regular schedule. Amount each feeding would probably be around 1 1/2 cups. If he absolutely refuses the food, do some research on a balanced home cooked food or consider raw. Boiled chicken alone is not enough and not balanced.