Hi Will,
Firstly, I'm going to ask that you take Pablo out of that tank, pop him in a tub of fresh dechlorinated water, and put him in the fridge.
Then, please give a run down on your tank parameters.
Have you tested the water for Ammonia? It sounds like it's a water quality issue. Water quality is most important to an axolotl's well being, if it is off this will make the axolotl very sick.
How frequently do you do partial water changes?
What size is the tank? Is the tank full?
What substrate are you using?
Can he close his mouth at all? Is there anything such as a stone which he could have swallowed and is stuck inside? Have you had a look inside the mouth?