My first dragon was about 8" at three months so yours does sound a little for it's age. You mention you feed it 12 crickets a day, that seems a bit low. What size crickets are you feeding it? Do you feed as much crickets as it will eat in about 10-15 minutes? How often do you feed a day?
He should be getting 1/4" to 1/3" crickets (the same size as the space between his eyes) and fed twice a day with greens offered at all times even if he ignores them. You could put the greens in a glass or ceramic container big enough to make it hard for the crickets to jump out but low enough the dragon can step over the sides (I use ceramic ashtrays (bought strickly as a food dish only) that have about 1 1/2 " sides) You can put greens in there then drop a few crickets ontop...hopefully in his mad dash for the crickets, the dragon grabs some greens to (greens should be finelly chopped).
Raise the temperature of the basking spot a bit as was already suggested and get a fecal done.
It is also alright for the dragon to jump off it's basking spot to attack crickets. I would however remove the sand and use just paper towel, newspaper, butcher's paper or plastic tablecloth as a substrate till he is at least 10" long to avoid problems with impaction.