I would try feeding him more frequently and better quality. I bought an axolotl that was an adult already, and could only buy bloodworms to feed her. She maintained the same size for about a year, which was around 5 inches long from tip of the tail to her mouth.
Then, I bought another axolotl who was rather small and I kept them apart for a little while until he grew to the same size as her.
Then, recently, within the last year, I was able to find them Red Wiggler worms, and Canadian Nightcrawlers and their size has grown substantially since the switch! They are both almost 7 inches long and are very healthy with fluffy gills.
The point is, feeding your axolotl something with little nutrition will not help it grow big and strong! It's the same as as us eating junk food apposed to vegetables when we are young lol. Try giving your axie a variety of things, I really strongly reccommend earthworms from your own garden (so you know they are not poisoned) or if you can buy some at the petstore.