His size doesn't sound too bad at all. But it doesn't sound as if he is eating much, 10-15crickets a day is not a lot for a beardie. They usually can eat as many as 50 a day.
Although this doesn't necesarily mean there is something wrong with your little guy, its worth looking at the situation to see if there may be something up.
Was he eating more, less, or about the same when he was in the smaller tank?
-I ask because sometimes when small beardies are in very large tank they get stressed from the size of it and feel less secure. This can lead them to eat less. They can also have a harder time finding their crickets. Where he is only 6inches right now i would suggest putting a divider (even just a peice of cardboard would work) in the tank, i would try giving him about 2/3 of the 55gallon instead of the whole thing. As he gets bigger you can give him more room.
What are his temps? Basking, and ambient?
-If the temps are too low (or sometimes too high) they will not eat as well.
Have you had a fecal exame done at a vet?
-If not i would bring in a fecal and get it read. Its always a good idea to do routine fecals (1-2x yearly) on all dragons anyway. Where he is a bit on the small size and not eating a lot, he may have some parasites. Fecals are pretty cheap and any clinic can do them without an appointment.
-If everything checks out (temps, negative fecal ect.) then i would just try to up the amount of crickets he will eat. Try offering small meals (10-15crickets) 3 times a day and see if he will eat them. Make sure he can see the crickets, especially since the tank is so big. If you have a lot of decorations you may want to take them out and feed him in an emptier tank. You may even want to feed him in a seperate tank eventually, but i would wait until he is eating properly before making that switch.
You can also try feeding from a dish to help him be able to catch the insects easier.
-Try adding some variety by offering other insects such as small mealworms, the occasional wax or butter worm, silkworms, or roaches.
-continue trying to get him interested in veggies but be more concerned with getting him to eat him protein right now.
Good luck!
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0.1 Bearded dragon
0.1 mexican Black kingsnake
1.1 Leopard Gecko's
0.0.1 Tokay Gecko
1.0 Blue Tongue Skink
0.0.1 Reverse Okeetee Corn
0.1 Anerythristic Corn
0.0.1 Red Tegu
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4.1 Cats