Wow, your axie is skinny.
couldn't see the bump, but it could be because it's so skinny.
My sister's older axies have a very low tail fin on their backs.
Are you able to take your axie out and put in in a container of dechlorinated water, and if too hot, put him in the fridge changing the water 100% daily.
Just that if you can increase the temperature of fridge to 9 degrees, it may be cool enough but will also allow him to feed if he wants.
If you can't keep in fridge then keep in a container doing the changes but only if you can keep the tmperature cooler than 20 degrees.
Just thought it might be easier to feed him things like frozen blood worm cube (rinse b4 adding to container as it fouls pretty fast); or live bloodworm if you can access some; leave an earthworm in the container with him (chop it to about a few 1/2 inch bits (just in case the worm is too long to swallow); try axolotl pellets - aquarium science or master foods brands. he might be more inclined to have a go if you drop it in on him and check back a bit later (this is how my sister got hers to eat gradually back to health - they wouldn't take handfed food when they were sick but would snuffle after the lid was back on).
I know that Australia has been having some rather hot weather, so if temperatures a concern, you could just take him out of the tank to feed him in a container or sink a bowl in (like some people do and leave some food in)
btw do you add anything to your tankwater and were your ammonia, nitrite readings 0 when you tested?