Do you have a submersible heater in the water? I keep my turtle tanks heated at all times because it's too cold in winter, in summer the air conditioner is on, room temperature is always too cool for them.
Maybe he's not warm enough.
I agree get rid of pebbles he can swallow & if he doesn't eat soon see a vet because he could be impacted or he may need to be tested for parasites.
They will stop eating with certain types of parasites.
Let us know what happens!
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PHRatz