I see other people have already given you good advice, but I just wanted to mention my own personal experience.
The most common reason for this is Wi-Fi interference. Wi-Fi and zigbee are in the same band, but Wi-Fi is a much stronger signal, so it can prevent is it the device from connecting. This can cause it to be marked as away then present then away then present, etc.
By any chance is there a Wi-Fi booster in your son’s room?
At our house we have a Wi-Fi booster. If we put it on the north wall in one room, and all the SmartThings zigbee devices to the west of it become unreachable. If we just move the Wi-Fi booster to the east wall instead, everything works again.
I also had an ongoing problem for over a year where my SmartThings presence sensor would start doing the away/return repetition every weekday afternoon around 330. I’m pretty sure one of my neighbors had boosted Wi-Fi and a kid who got home from school about that time. 
I worked with support and we tried everything, including increasing the delay before the device was marked away, but it didn’t help. I ended up not being able to use that device at my house.
Anyway, the first thing to do is to change the reporting interval as @csc suggested. After that, look for sources of Wi-Fi interference which you do control. See the following.