A mix wouldn't have anything to do with him being skinny. He can have a different shape but the fact that his bones are showing have nothing to do with his breed.
From the pics he doesn't look mixed. But I'm not an expert.
Kirkland is not the worst dog food but not the best either. But many dogs keep their weight even on cheaper kibble.
I'm not sure. Maybe someone has some ideas. If you feel like he doesn't like his food then try feeding real meat for a while and see what happens. But if you're saying he eats normal then to the vet.
How much do you give a day?
ETA the pics are not the best but I think I see his back bone.
Are his ribs visible? On the pic they're not
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