Hello there. Sorry you're worried about Gizmo (cute name). How old is the little one? I do agree with Karrie about the lighting, it could have some affect on him since tube UVBs are the best for a growing beardie and for your tank size...But can you go over the size of the tank with us too?
My beardie is currently doing the same thing. She hasn't been eat much since the beginning of July, but that's just how she is for a year and 7 months old. And I'm sure others can weigh in with how their beardies act as well. But it seems to be a pattern with mine, that around this time of year (living in CA) that she acts this way and I've learned not to be alarmed. Sometimes their appetite just changes and they go through unnerving phases.
I'm sure little Gizmo is just fine though