I think he's nearing the age where he might hunt down things like pellets, but it will be a trial and error thing. I think you could leave the pellets in the tank for about 20 minutes or so, maybe a bit longer whenever you want to try. He's also at a great size for earth worms. Red wrigglers and Canadian night crawlers are the most recommended, the latter being the most easily accepted because the red wrigglers release a foul tasting slime. If you wanted to go with worms, which are the most nutritious, you would feed them a worm about the length of the axolotl. You could also cut them up to make it easier for the axolotl to eat them, but mine will eat them whole (when he does).
Glad to be of help, good luck with the food. My axolotl's favorite are Hikari sinking carnivore pellets, but I've been attempting to switch him to earth worms as a main diet. He seems to like the Canadian night crawlers more than the red wrigglers.