As mentioned above, some beardies just won't eat their greens when young, while some eat them like no tomorrow at a young age. Try offering different types of veggies than you are currently offering, might be he just doesn't like em.
To be honest, I would not stress over it one bit and try not to force the issue if it is a juvenile dragon. They need their protein from the live feeders and that should be their main diet. Offer a small portion of fresh veggies daily, but make sure they get the live feeders 2-3 times per a day. As your beardie gets closer to adulthood, then you can start a regime of only offering salad in the morning time and then live feeders later in the evening. For now, if he doesn't want the veggies then don't worry about it. He will come around in time.