kiosk mode – Single-purpose Android Tablet for handicapped person

My brother is profoundly disabled. He enjoys working word search problems. I live 1400 miles away from him, and I only get to visit him once per year. During that time I install different word search apps on his Fire tablet. But within a few days, something happens and the Fire sits unused until my next visit. What I would like to do is create a tablet that only runs the word search puzzle, and nothing else. I would like it to boot in this mode, and run only this app. Ever. Until the tablet dies for good. He has an effective IQ below 60, and he cannot understand ads, other apps, complicated boot up sequences. I have heard about the kiosk mode, maybe that would work, but I have not been able to understand how to implement. Ideally, I would buy an inexpensive tablet, configure it to run one wordsearch app, and send it to him in the mail. And for the most part, never need to worry about it. It would be a “tablet as an appliance”. Could I please get advice on how to do this? I lament my inability to make a tablet work in a way that would let my brother tap into the larger potential of Android, but just having a single app would be a game changer.