Is there any AI with extremely high sensitivity to impaired speech?

Hi everyone, My brother has Down syndrome and is autistic. He is minimally verbal, and his speech is significantly impaired. I had the idea of building an app, almost like Duolingo, with a gamified experience: it would present a word, he would try to say it, the app would listen, provide feedback, and gradually increase the difficulty as he improves. The biggest challenge is that his speech is very difficult to understand. My family understands him because we've lived with him for 12 years, but almost no one else can. Speech-to-text models like Whisper and other AI systems almost never recognize what he's saying, so the app wouldn't work as intended. Do you know of any AI model or speech recognition system that is sensitive enough to handle speech like this? Or perhaps another technical approach that could work? Thank you!