I agree with the others-for a Warmblood of his age and size,he looks fine Don't worry! Just keep on with appropriate feeding & work regime for his condition score (at any given time) and age and he will fill out&muscle up soon enough - he's very handsome by the way and looks a sweetie! Hope you have lots of good times with him.
Not read the other replies but think he looks like our previous lad when we got him - big rangey TB x who was 5 and had not yet built any muscle. I could be way out but he looks ok to me - just on the lean side? I would "think" (no expert) that the right work will change him over the next year or two.
He looks fine, just needs to start building muscle through work. Please don't put any more weight on as you will be opening a can of worms.
He is stunning.