Try giving him a variety. I give mine many different insects/greens and fruits. If he dosn't eat his veggies, try dropping then from the top of his enclosure to show movement, that they are attracted too. Feed him occasional waxworms as treats if he cooperates with you also they love blackberries so use those as treats
I process the fruits, such as cantaloupe, with the leafy green veggies until it becomes finely chopped. This way your bearded can't pick out his favorite fruits... Gradually, over time decrease the amount of fruits so he is just almost eating the greens all by itself. You can add red bell pepper to make the salad look appetizing to encourage him to eat. Make the salad as brightly color as you can. Try using things slike winter squash, carrots and red bell peppers to give it nice colors.