Can't upload or delete apps on child's apple watch

My son has epilepsy and we were prescribed the epiwatch core app to track and alert us to seizures. His watch is paired with my phone and I have the app on my phone. However, I am unable to upload this app or any other apps to his watch--even basic ones like O2 levels. I have turned off screentime limits, have allowed automatic app downloads and made sure I don't have any updates to do...but so far nothing. I can't delete nonsense that is on there (non-native) nor can I upload what I need. For us, this app is literally lifesaving and the frustration with not being able to upload it after days and days of troubleshooting--it's exhausting.