I have a Red setter who has the same problems, he is now 4, and is still on the skinny side.
I found over the 4 years I have had him that dried kibble did not work for him, the more I gave him to help him put on more weight, the more the stools got loose and he was still hungry.
What to do, a very unhappy hungry dog who could not think about anything else. I put him on a food a called pure , a dehydrated food that you make up with warm water. He was a good food , but Copper still was thin. 20kgs.
There was nothing else to do but be brave and put him on raw. He did put on a but of weight but he was dealing with raw vegetables.
So I tried a company called butternut box, which is a cooked meal and vegetables, he did not put on weight to start with, so I increased the amount slowly and after a while he was on 900g per day,he is now putting on weight, slowly but he is not really digesting the vegetables, they come out as they went in, and I could not afford the every expensive bills. He is a dog with a sensitive tummy , and I have now learnt that he has a very high metabolic rate.
I have now found out that when you feed raw you do not have to always feed vegetables at every meal. So he is now from Sunday going back to raw and having one meal with meat and one meal with vegetables.
I have also tried to limit his running off lead as he is very energetic and will take off like a rocket, running after deer is his favourite.
I get very fed up with people saying that your dog is very thin and almost thinking that he is not being fed enough , but one day I hope that he will put on weight. I hope that you can help you dog put on weight. He is very handsome.
For people thinking that I have changed Coppers food a lot, sadly one dog food does not suit all and sometimes you have a real job trying to find something that suits. It has been a real eye opener and a lot of research has gone on.