Armchair vet here, but maybe he has some sort of iron/mineral deficiency and his body's natural reaction is to eat rocks?
I think when dogs are young they'll chew on darn near everything. He's at an age now where you should be able to correct it. Assuming you've introduced it to an e-collar probably, I'd do some "trash breaking" as they say.
In the simplest version of this, you associate the "rock" with "bad." But you do NOT say anything. You do NOT let the dog know you're anywhere in the vicinity. This can't be emphasized enough.
Get the e-collar on your dog and let him wear it long enough to forget it. Then let it outside. Ideally in an enclosed area where you can see it, but it can't see you. Be sure you have that collar cranked to a much higher level than you'd train with. You get ONE maybe TWO shots at this, that's it.
Keep a close eye on Fido, when he grabs a rock (you must be absolutely, positively certain that's what he has) and he's about to chew and/or swallow him, hit the button.
Now, if done correctly, your dog is going to have some serious enlightenment. He may jump, yelp, run, etc. But watch him. Don't say a gosh darn word and don't let him see you. In fact, don't let anyone, period, be near the dog. It might be all it takes, but chances are ol' Fido will give it a second go. If you haven't maxed out your collar, take it up another notch. Be sure the dog grabs another rock, and at that instant hit the button again.
You may need to do this a few times over the course of a couple days. But when done correctly, you could get the desired response with one session.
I should preface this that if you're a novice trainer or if you're unsure about this, talk to a veteran dog handler/trainer about "trash breaking." This has been used in the business for years to get dogs from chasing undesirables while hunting, such as deer, skunks, coons, etc. But I've used it on my own dogs to stay out of the garbage, and it works.
Got any questions, let me know. It's kind of a delicate things, and some folks might not be able to bare that you're going to hurt poor Mr. Fuzzywumps. I see it as I'm doing the dog a favor, and avoiding costly vet bills or worse.