Is an app appropriate for this? Have you looked into speech therapists? It sounds like you need a professional who is trained for this sort of thing. Not everything can be handed off to an app.
No app, parents and caregivers have to become the therapist. Depends on your child's age but these are the resources we used:
https://teachmetotalk.com/ https://www.hanen.org/
> The closest app is $200 just to show him flashcards and says an audio clip
Could you please provide the app(s)? We use this one every now and then: