See the monitor thread I linked to 
I’ve found the Orange Pi Zero LTS with an external (CSR 4.0) USB Bluetooth dongle is much more stable than a Rasperry Pi. It also has the advantage that you can install the OS on the eMMC, which has a much longer life than SD cards. You can run Home Assistant on anything, but performance is better on better hardware, than on a Pi.
You’ll likely find that two or three will be enough, depending on the property construction. I find that in modern construction (plasterboard/drywall) you’ll get a reliable range of about 10 meters indoors.
Add in door sensors so that you’re only running scans when it’s possible for somebody to have left, and you should have high confidence detections.
You could also use a camera with object detection (like Doods). Even without face detection (which clearly doesn’t work when somebody is walking away) it’s another potentially useful data point. Motion sensors are potentially other useful data points here. For example:
- Motion in the hall
- Front door opens
- Front door closes
- Person detected
Now you know somebody is leaving. You don’t yet know who but if you can determine the presence of others, you could provide a low confidence alert that the son has left unattended. Even without the camera you could do logic with the hall motion sensor not being active after the door closed. Multiple motion sensors increase your confidence.