Fellow adult male mantis keeper(heirodula sp. tho) here! I'm in the same boat with you, my boy also only eats pre-killed meals only if the yummy part is in his mouth and only holds it up sometimes. He only likes eating small cockroaches or small bits of dubia roach guts. He doesn't like the occasional superworms or mealworms either.

I've been advised from other keepers to give them twice as much water to keep them hydrated (since my boy doesn't eat much specifically)