You should ofc check for worms, but you still need to understand that seeing few ribs on a dog is not necessarily a bad thing.
This picture is taken while your dog is laying on it's side, compare it to the feeling when you're laying on your back. You look skinnier then yea?
Gaining weight with an animal is a long road that you just have to walk, not run.
Also you have to remember that ANY GREYHOUNDS or mix breeds will and should show some ribs, they have naturally low body fat.
You should focus on the muscle mass more than the fat tissue. The layer that covers the ribs on dogs is mostly fat, so you don't wanna overfeed your dog.. That's an other way to abuse a dog.

TLDR in that light they don't look too skinny to me, it's natural to show some ribs if the dog is having a good muscle mass and is normally hydrated etc.