@Barney79 

It could be, because:

1. Your son's mobile is not connected to Deco mesh, but uses cellular data or connects to someone's else WiFi network;

or

2. Your son configured static IP address on his mobile. Device with static IP address configured on it may not be visible to Deco mesh.

There is also an option to bypass Deco parental controls by enabling "IP address randomization", which is set on by default on most modern smartphones. Yet, if this is the reason, you should see some unknown devices connected, which you can't recognize but they are visible in Deco app. These are from random MAC addresses generated by smartphone. 

Usually, this is the most common case, but if you are confident you see no unknown to you devices active or inactive in Deco app, the reason must be one of two I listed above.