You're going to get a wide range of answers from a question like this ... but here are a few things you can do to help.
I would get him on a regular feeding schedule with his salad. I feed mine a daily salad and give it to them fresh, first thing every morning. They have learned to rely on that and will even look or stare at me funny if I'm late with the salads! LOL I've been known to sleep in a little on the weekends. 
Alternate the greens, veggies, and fruit every week so he doesn't get bored with his salad. Make sure you are sprinkling his salads once or twice a week with a calcium supplement (like Miner-al) and once a week with a multivitamin like Herptivite.
Crickets are good, and occasional mealies as treats are ok too. But have you considered silkworms? They are much higher in calcium and beardies really love them. I know they aren't as cheap as crickets and mealies, but the benefits of feeding them to your beardie are amazing. Silkies are what I feed to my malnourished and skinny beardie rescues to fatten them up rather quickly.
Keep him hydrated. Offer him juice diluted with water, or Pedialyte. You can add all sorts of stuff to the juice, like acidophilus and vitamin B - both should increase his appetite and are not harmful to him.
Make sure your environment temps are correct. A beardie that is too cool won't digest food properly and will most likely have a decreased appetite.
Check for parasites often. I check all my beardies every 3 months. Parasites can be very detrimental to a beardies health. They can steal the nutrition that the beardie should be getting and they can cause other major health problems as well.
And lastly, I would make sure that you have a good UVB source. UVB rays have proved to be essential to a beardies health. Proper UVB can increase a beardies appetite, aid in calcium absorption, and can increase the overall color intensity of your beardie.
Good luck with your dragon! And don't worry about his size ... Being healthy is way more important than size. As long as he's healthy and parasite free, he will grow with time.
~Veronica
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Beautiful Dragons
