Besides dusting, the appearance of an injected bug is not different from the appearance of a non-injected bug (besides maybe being a bit fatter from the injections). Maybe he is just being picky about the type of bugs.
Try feeding him different types of bugs. I have a cricket, roach, superworm, mealworm, and a roly poly colony (attempting over and over again) going, so that my chameleon can get many different kinds of bugs. I also go catch small mantids or katydids that I find, as well as hornworms I find on our tomato plants and grasshoppers. He isn't really a picky eater. He also gets the occasional white lined sphinx moth!
He doesn't care if it's dusted or not. In fact, he likes the dusted ones the best because he can see them very well against my feeding cup or my hands.