[AbbyK9]

This looks like a relatively young dog to me. Young dogs tend to fill in as they get older, so I would not be too worried about pushing weight gain at the moment. Looking at his waist and ribs, he doesn't actually look too thin to me, especially considering that sables do tend to show contours more (and therefore tend to look more ribby than they actually are) because of their fur color.

What would concern me more than putting weight on the dog would be putting some muscle mass and definition on the dog. He looks like he has very little muscle in his hind end, which may contribute to him looking so "skinny".

 

[AbbyK9]

Yes, he was so starved that he started to atrophie according to the vet. What would you suggest for adding muscle? He will be one years old the end of this month

Since he is so young, I would not be in a rush to put any weight on him at this point. Even though he was starved, most German Shepherds will naturally go through a lanky phase at this age and appear skinny and not well proportioned. Once he gets older (two years, three years) he will start filling out more, so there's no need to put a lot of weight on him now.

As far as putting muscle on him goes - exercise will help with that. Whether you get into a dog sport or just spend time going on long walks, runs, etc. anything would help getting his muscles built up.