I'm new to this forum, and I don't consider myself an expert on dragons by any stretch, but I might be able to help with your question a little.
There are a couple of things that may be going on with you little one. If he is 8 inches he is still pretty young, and it is not uncommon for dragons to develop more of an appetite for veggies as they mature into adults.
The other possibility I can think of, which may be more likely, is that he is still adjusting to his new environment. While crickets may be "yummy" enough for him to chase around a new enclosure, it may take him a little longer to be interested in his veggies. If possible find out what kind of veggies his previous owner fed him, and make sure to offer your dragon many different choices if you can't figure out what he was eating before.
As an example, one of my girls did not eat her veggies when I first brought her home. I contacted the breeder and found that she feed her dragons dark leafy greens like kale and collards without many veggies. The mix I was feeding mine were spring greens with yellow squash, zucchini, peas, and peppers. Because the veggies looked so different to my newly added dragon, she wouldn't eat them. The next day I bought a bag of collards, chopped them in the food processor and served--she ate them right up.
I try to add a little bit of kale or collard in with my mix now, but once my new dragon was more comfortable in her environment she started eating the same veggie mix as everyone else.
I hope this helps a little, and best of luck with your little one!