Also, for the worms, you won't need to cut them up unless he's never eaten them before and may struggle with the size or if they're bigger than his mouth - mine can eat worms as long as their body and keep them down, but they're quite experienced with them now.
If you need to cut them, use scissors for a quick and clean way to feed your bubby. Worms are ideal if your axxie is poorly as they're packed with nutrients and the wriggliness often sets off the neonatal desire to eat.
If he still won't eat, don't worry overmuch. He'll be in cooler water so he'll eat less, plus if he's a bit stressed he won't eat for a bit. The main thing is good water quality and cooler water for now.
I'm inclined to think the arched back is due to stress - as far as I know axolotls have very bendy bones that are very difficult to break so I don't think it's broken, but obviously watch him carefully and if you see any signs of pain then a vet trip may be in order. If you see shedding skin (not uncommon after falling out of a tank - Gally shed when he jumped out of his tub once) a tea bath will help to keep him calm and keep any fungus out.
Hope this helps!