how old is your cham and what species is it? If you've had it for 3 months already I'm going to assume its at least 5-6 months old already. Besides the obvious pygmy chameleons and a few other smaller species, most chams should be able to eat at least 1/8th inch crickets by then, if not bigger.
Try to feed him other worms like pheonix worms, silkworms and hornworms. Only feed him mealworms on occasion. Their hard shells are hard to digest and can easily cause impaction.