Hi Nicholas,

Acid in the stomach will not eat away the hook.
If you're REALLY lucky, the hook may pass in the stool. If your dog went already and the hook isn't out, that means it's still inside.

Have your dog see a vet ASAP, today if you can because even if your dog isn't showing signs of obstruction, the later affects could be quite horrible.
It takes a while for animals to show signs of something being wrong. The key is NOT to wait. If you know he swallowed it and you know what it is, you're at a great advantage. I've had dogs seen at the clinic where they were not eating or drinking for a week before they were brought it. By then it was very hard to bring the animal back around. One case, the dog swallowed a jagged bottle cap and it caused serious damage to his intestines. The hook has a much greater chance of attaching itself on something inside.

Your vet will do an xray and it's a good idea to do blood tests since this will tell how his body is functioning, whether there's a possiblity of toxins in the blood stream.

Your dog is fine now only because it just happened. Don't wait to bring him to the vet!

Good Luck and please let me know how it goes.
Olivia