My now adult crested gecko used to look just like yours as a hatchling- rubbery and frail. My vet diagnosed him with metabolic bone disease and gave me a liquid calcium supplement to mix with his food.
I'm not sure if it was the reptile shop's fault or if it's possible that the mother didn't get enough calcium. Anyway, my best advice would be to get him into a vet or try to find a similar supplement online.
I can happily say that my gecko's a normal looking adult these days and has made a full recovery, so there's hope!