hmm they do appear to have reduced which could indicate an issue with water quality, temperature or disease (fungus). Have you seen any white cotton like fungus on his gills?
Either way the best course of action is get him out of that aquarium and away from the pebbles for now.
If he has had an issue with fungus, the course of action to take is salt baths (See link in my sig).
I cannot stress how important it is to test your water, and make sure it's ok, before putting him back in.
Then if all is well and the tank is not yet cycled perform daily water changes until cycling is complete.
As for whether he's eaten any stones, it's hard to tell unless there are lumps or other issues like him not eating, floating, being sick etc.
Shine a torch on him when in a shallow container and you may be able to see a silhouette of a stone.
If there are no problems indicating that he has swallowed some, then don't worry, you may have dodged that bullet, but get him out before it can occur.
If he has, then a period in the fridge may help him pass it or regurgitate. However I wouldn't do this unless you can be sure there's an issue in the first place.
I hope that helps, we'll get the little fella right as rain
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