From the pictures he looks ok to me, but if you are concerned maybe he is too thin. It's been my experience with Giants that in the first 2 years or so they grow taller so fast that they just don't have a chance to fill out.
But my 3 yr old St./OEM X has a weight issue, we, like you, have tested for all kinds of things, and he never looks quite right. In build he's a lot like your dog. Unlike your dog, you CAN see his ribs (barely) and he's very light in the rear end. I boarded him at the kennel where I work recently, and had them give him Eukanuba Low Residue, as I was out of his food, and had no time to go to the store. Well, they were at the kennel 5 days, and I swear his ribs are less noticeable! Low res is a veterinary formula, supposedly for sensitive stomachs. I'm arranging to order some from work, it might be worth a try!