Some can be very picky I had a small asian toad who seemed to have been only raised on meal worms and crickets. First time I fed him a small earthworm, he attacked, stuffed it in his mouth, and - if toads could - looked utterly shocked, spat it out and would actually avoid or flee earthworms ever since lol

You can gutfeed mealworms so they are a better food, but you're right: Basically they're not the most nutritious food.

A trick I have used that you can try, is this: Gather some small earthworms (breeding them yourself is easiest - the babies will be small and easier for him to catch and you won't have to cut them up), small critters from the garden or small phoenix worms.

Feed him one meal worm, then try immediately with f.x. an earthworm. Sometimes when they get exited about the food, they "forget" to be picky and just attack. Otherwise gutfeed some small crickets and cut down on the mealworms and leave a small amount of crickets in his enclose (not he black breed, that eats meat). Just like 4-5 of them that are his apropriate feeding size (the distance between his eyes).

Put them in there by dusktime and keep an eye on how many are left next day. Feed minimum of mealworms and keep trying. Patience is key

Good luck!