My favorite puppy food is Nature's Variety Prairie Large Breed Puppy. It's available at Petco, and most any pet specialty/feed store. It has really good ingredients and it is made especially for large breed puppies, to ensure that they grow slowly and gradually rather than in spurts. Nature's Variety is an awesome brand, and you can start mixing Instinct into his food at 6 months then transition him to grain free when he's done growing.
Another brand that I like for large breed puppies is Natural Balance. It has good ingredients, and a suitable protein/fat/calcium/phosphorus balance for a giant puppy to eat... and it's good for all life stages! So you'll never need to switch foods unless you want to. They have grain free and grain inclusive formulas. I would feed either Ultra or Alpha Dog (grain free), or you could even go between all of the formulas for variety!