I've probably got not much new to add. I agree with the others that your dog doesn't need puppy food.
As a large breed, yes they grow over several years. But the nutrition levels in adult food are adequate for growth, and more suited to the longer slower growth period that large / giant breeds have.
Mine are a large/giant breed and our pups have puppy food until about 4-6 months old, with raw mince that includes bone material. They will continue to grow until they are about 4 years old. All this longer growth period means is that they may be eating slightly more than an adult, spread over two meals. But they won't be eating food with different concentrations of nutrients.
Just like a human teenager will eat more than an adult, but the same sort of foods.