I agree, definitely move the axolotl out of the tank, and try keeping him in the tub for a while.
When you have the axolotl in the tub, can you take some further photos of the axolotl from above (birds eye view)?
Very lightly and gently probe the lumps with your finger, do they feel hard (like rock) when you touch them?
I would suggest keeping the tub in a cool and dark place for a few days, and see if the axolotl is able (and wiling) to eat while in this still and stress free environment (I can see some possible problems in the tank which may be causing a loss of appetite. I'll address further down the post). Check the following morning to see if he has kept the food down, or regurgitated it. While the axolotl is in a tub, it will be much easier to monitor what goes in (food) and what comes out (wastes), than in the tank environment where the evidence is lost to the substrate and filter.
Give him a few days in the tub, and if he still doesn't eat, or is regurgitating everything, you can try fridging to see if he is impacted.
If those lumps do feel hard as rock though, I would fridge him straight away.
Fridging will help the axolotl pass any stones he may have swallowed, it does however also slow the metabolism right down, so you will find that he will not eat much (if at all) whilst being fridged. On a good note, the weight loss is also slowed right down as well though which buys you some time.
While the axolotl is enjoying his holiday in the tub, this gives you time to identify and correct any problems in the tank environment which may be contributing to his inappetance.
Common problems:
- Water quality issues. I know you said you are not testing the water, but, are you doing sufficient partial water changes? How much water do you remove and replace, and how frequently do you do this?
Small water volume can make it hard to keep the water parameters in check. You need to be quite diligent in removing all wastes and excess food (axolotls will also on occasion regurgitate their food) and this can be difficult when you have large rocks as the substrate. A lot of muck can accumulate and get trapped in nooks and crannies under the rocks and this can wreak havoc with water quality.
As Caitie has already pointed out, the rocks also present a great danger of impaction. Axolotls are more than capable of swallowing very large rocks indeed. Have a look through these threads:
http://www.caudata.org/forum/f46-beginner-newt-salamander-axolotl-help-topics/f48-axolotls-ambystoma-mexicanum/f58-sick-axolotl/68478-large-rock-belly.html
http://www.caudata.org/forum/f46-beginner-newt-salamander-axolotl-help-topics/f48-axolotls-ambystoma-mexicanum/f58-sick-axolotl/73204-axolotl-surgery.html (have a look at posts 16# - 18#).
- Too much light. That tank light looks to have very high intensity. Axolotls don't care at all for light, as they have no eyelids to shut it out if it's bothering them. Which brings me to hides...
If you are keeping a light on in an axolotl tank, you will need some dark places (hides) for the axolotl to retreat to if the light is aggravating them.
- Water currents. Is there any water flow being issued from the filter? Axolotls find water movement to be quite stressful, they prefer still water. Water currents can stress them enough to put them off their food.