Do you know what substrate he was on before you got him? What do you have him on now? In case you don't know, gravel is not recommended for use with axolotls as they can attempt to swallow anything that is not larger than their heads. Other parameters would be good to know too - ammonia, nitrite/nitrate, temp. Is your tank cycled? If not, you will definitely want to keep a close eye on all the parameters until the cycle is complete.
My first guess would be that he has swallowed some gravel as the stomach is very dark and swollen. If you shine a light through his stomach area, you should be able to see if those are stones, gravel, or something else. Depending on the size of the gravel/stones, he should be able to pass these over time.
If this is the case, you can fridge him for a while to help him pass the stones/gravel. If the stones look large, you will want to consult a vet.
If it's not something he swallowed, it could be an infection or possibly the beginning of bloat - a better pic could help in determining this. If this is the case, you should consult a vet. However, if it is bloat, I'd contact your seller as to the best of my knowledge, there isn't a real cure for bloat.