Yeah, he's definetly ready for adult food! Or even a high grade food that's designed for puppies and adults.

About you saying that your dog was eating less on another food, well it depends on what's IN the food. If it has better products in it, then yeah your don't wont have to eat as much to get his full or to get all of his energy that's the way it's suppose to go theoretically.
Plus he's getting older and not growing as much as he was before, therefore does not need as much food.

Natural Blend looks to be a good food, don't know too many people who have fed it, but I've taken a look at that food's ingredients and what not many times over.