To help battle the anemia, start feeding him chicXXXXX XXXXXver for two weeks. Boil the liver as the method of cooking. Feed one once a day of the liver as a supplement to his regular diet.
If he doesn't want it, blend it up in a blender with water or tuna juice. Make it soupy and try to use a medicine dropper to feed it to him.
Liver is loaded with iron and vitamin B12. That is what he needs right now. This will help him produce more red blood cells.
Hopefully once the red blood cells are back up, his appetite will return.
To help put weight on him, you can start mixing kitten food in his regular food too. Kitten food has more calories and calcium in it than adult cat food.
I hope he starts feeling better soon.
Best of luck to you,