Ok firstly your gonna have to get him eating otherwise he won t get any better try mashing up some bloodworm and force feeding via a 1ml syringe do that as often as he used to eat then keep an eye on him for a few days if still no sign of healing you could try some sanipur from tetra its a hebal treatment for scratches and bites your local reptile shop should have some or can get some, im not sure if this would work with newts but form my injured frog i got told to use vetadine http://www.eurorep.net/medivet/vetadine.htm

let me know how it goes, good luck