Transitioning him to kibble won't be easy, so my suggestion is to feed a HIGH quality canned food like Merrick, Nature's Variety, or Evangers. It will smell like the "fresh"
food he is used to eating but much more appropriate for him and easy to measure the proper quantities. I highly doubt he would pass up something like Merrick Wingaling.
You should definitely go to the vet. They can see if he has any underlying health problems from being so overweight and give you a healthy weight to aim for.
Good luck!