You can try crickets, with the back legs removed. Those are pretty easy to get at about any pet store you go to. They can get tired of a particular food item, but I think the main issue here is that his conditions are off. Usually when my girl 'food strikes' for another type of food it doesn't last more than two or three days.

You really need a digital thermometer/hygrometer. The kind you have is VERY inaccurate. I used to use those, and when I got my digital one, I found out that the thermometer was about 10 degrees off and the hygrometer more than that.

I think your substrate is probably WAY to wet at the moment. It looks pretty water-logged in that pic, and if your hygrometer is reading 90% at the top of the tank, it's probably too wet at the bottom.