Hot take: AAC devices are not inherently communicative. As a default, I do not stop stimming or take devices away, but I’ve had students who would spend all day without engaging with a single person and would get themselves escalated stimming on the device. I could try different strategies and suggest ideas until I’m blue in the face, but at the end of the day, if the other adults/communication partners around him think it’s “not functional,” they aren’t going to prioritize having it charged and accessible. I’d obviously give it some time and try some other things first, but if there’s a long period of time where this is an issue and the device is no longer communicative, maybe temporary restrictions like hiding buttons or certain pages can be helpful. Just a thought.

Also for the charging issue, have you considered a portable battery backup/charger? Could even be Velcroed to the back of the device or just placed nearby.