If I remember correctly, he is about the same age as my boy (18 months). Stark is just starting to fill out a bit... heavier in the chest, neck thicker, etc.. and I just noticed this change a few days ago. He is still VERY skinny in the back though, you can feel his ribs and spine when you pet him. He eats 3.5lbs of food (raw) per day plus training treats and he still looks skinny/gangly. I believe it really is their age and development playing a part here. They are in the 'inbetween' stage, not yet a man, not quite a boy'.
He looks great. Nice and muscled for his age, lean, active, etc.. Keep doing what you are doing.