Fire TV out of storage

Even if the apps are stored on the SD card; they still can store some info on the internal memory... this will also slow your system way down because the system uses the same memory for both storage and "ram." So even if you have it set to automatically put stuff on the card, it will try to filll up the internal first, which will slow your system.

That being said, try clearing out the cache of the programs within the app manager. Also make sure that if he's downloaded anything that it actually downloaded onto the card and not the internal storage (that's a bitch of a problem I had with Kodi). I've read that the Amazon Prime Photos app can be a storage/memory hog, so you might want to check that out too.

I'd recommend getting an app like ES file explorer and using the analyze/cleanup feature to clear out some of the bloat.

Settings->Apps->Managed Apps->Kodi->clear cache then force stop

Also, does he download movies before watching them?

Found out he had saved 4 copies of the Kodi data, each around 800 MB. I deleted and set him up to backup every week to SD card. I also showed him how to clear cache. Got him down to 3.2 GB on the internal storage. Running smooth again.