He doesn't look too thin to me, some dogs are just naturally lean. My Toby is the same way, I get comments that he is too thin all the time, but the vet says he is perfect. Has your vet told you that he is too thin? The rule of thumb is that you should be able to feel his bones easily but not see them. If you could post some additional pictures from different angles, maybe we could tell more. But it is easier to tell in person.
If you are worried about it, Ziwipeak is excellent and I've only heard of a couple of dogs that don't like it. Toby is a picky eater, but he lovesss his Ziwipeak more than anything. You could try that, although I do not know if it is available where you are. Maybe try bully sticks- they are a big hit here and add a few extra calories. Also, you can try dehydrated chicken or other poultry or dehydrated liver. Many dogs love that.