How big is he? I am not sure how quickly fire bellies grow, but I would think at 6 months he should be eating something bigger than fruit flies. More like small crickets. The small foods may not be enticing anymore. Make sure the foods are no bigger than the space between his eyes, but big enough to be fitting his growth. I've also heard that live blood worms can do damage in the stomach of an animal that doesn't chew...that could be just a rumor, but maybe look into it.
I would read through the care sheet and make sure that all your bases are covered - Frog Forum - Fire-Bellied Toad Care and Breeding - Bombina orientalis and relatives - I wish I had more experience with firebellies to give you more help![]()