Your axolotl is a bit thin, I'm sure it won't take long to fatten him up though. As a rough guide, the axolotl's belly should be about as wide as the head - plump but not obese.
These articles give a good rundown on the best foods for your new friend, earthworm is the best as it has all the nutricion an axolotl needs in one wriggling package:
http://www.caudata.org/cc/articles/worms.shtml.
If you are reluctant to feed live food to the axolotl, there are non live food options as well:
http://www.caudata.org/cc/articles/foods.shtml
Cheers Jacq.