My bearded dragon is not eating !?

My bearded dragon won't eat any bugs or worms, if I leave them in the viv he will ignore them and if I tweezer feed him he just looks away. He hasn't eaten for about 3 weeks now and is losing weight. He isn't very active but sometimes kicks his substrate about into his food bowl. I offer him a number of veg including broccoli, carrots and banana skin and apple. Do you know what could be wrong or how I help him to eat more and gain weight ?? Thank you !