This may partly be because of his age. He is still young and should be eating mostly insects. By the time he is 18 months old, however, he should be eating 90% greens and 10% live food.
I got my beardie as an adult and he loved his salads from the start, but I have discovered some things that make him more eager... its worth a try anyways.
-Mist the greens. My boy seems to like "dewdrops" on his greens.
-Hold the dish in front of him. Instead of putting the salad bowl in a corner or somewhere where he may not notice it right away, hold the dish in front of his face while he's basking. If you use a steep-sided dish, you can use a tupperware lid.
-Change salads every 24 hours
-Change salad an hour or so before you usually feed live. This is the time when he will be the most hungry.
-Add worms to the salad (if your beardie likes worms). Then hold the dish in front of him (as mentioned above). He will see the worms and go to get one, also getting a mouthful of greens. Then, even after he has eaten all the worms, the greens will still smell slightly of worms. He will also get used to being fed worms in that dish, and when you put it in the enclosure he will get excited.
-Sprinkle frozen mixed veggies (the brand I use includes corn, peas, carrot bits, and green bean bits) on his salad. My beardie adores frozen mixed veggies. Apparently, they have some preservatives and salt in them that make them not as healthy as fresh greens and veggies, but if you combine them with his salad and he likes them, he will end up eating some greens, too. If this works, gradually decrease the amount of frozen veggies you add.
Good luck! Don't worry too much; in the wild, baby beardies eat mostly bugs.