if many small rocks build up it can cause impaction so for now you're going to need to keep a close eye on your axolotl. It may require fridging if it doesn't pass the stones on its own. Offer lots of food to help the gut coat & pass the gravel - if he goes off food, then you need to worry more & think about putting into fridge.
Please follow Nikki's advice and remove the small gravel. You can keep it bare bottomed, replace it with sands <1mm in diameter such as pool filter sand or playsand (amongst some other sands), put slate down or use large rocks 5+ cm in size instead. I hope it passes the small pebbles without a problem!!
called up pretending to be someone wanting to buy an axolotl & asked her what I'd need to buy to set up a tank... She went through 2ft tank, filter, maybe air stone & gravel! I said I'd heard gravel is bad & her response "it's fine if they don't eat it!" - yup that did it - I lost it & told her I'm not wanting to buy one at all & that this was a confirmation call to see if they were as incompetent as I had been reported. I then asked to speak to the manager... Got hung up on! Called back in 15mins to speak to Rhonda their store manager & she "didn't appreciate being told how to do her job" I tried to remind her of her obligations under animal welfare laws to not keep animals in unsafe conditions! She said she couldn't do sand or bare bottom for their display because its an under gravel filter!! I got beyond angry & told her to expect the RSPCA as I will be making a full report! that's soon going to change - I will soon be known as "That annoying axolotl woman!" very soon if I'm not already! lol