My dog is 8-9 he won't eat, he eats grass! What should I do?

Dogs eat grass for many reasons, sometimes it's to bring up a hairball, or to settle their stomach etc.

If he's not eating his food, but he is eating grass, than there's most likely something wrong and he's eating the grass to try and remedy the ill.

How long has it been since he's eaten?

Is he drinking?

Has he vomited? How are his bowel movements?

If he hasn't eaten for more than 2 days, it's time to go to the vet. Something could be lodged.

Please keep us posted. I hope everything is okay.

Do you mean your GSD is 8 or 9 years, months or weeks?

Healthy dogs won't starve themselves, and like Alty mentioned all dogs will eat grass from time to time to settle any of their stomach issue.

Again like mentioned, if your dog hasn't eaten for more than 48 hours, you need to seek a vet, there are too many possibilities to make a guess.