Cumulative Internet Usage Blocking - Parental control

I am trying to limit my son's daily internet usage to 2 hours. How can i do that on automated basis? My ISP gateway allows me to do it where as firewalla is missing this important feature. Let me know if there is a workaround for it. 

Here is the situation - The son has 2 devices - 1 Tab and 1 laptop. I have added both for the user there should be a way that if internet usage for this user exceeds 2 hours. Internet gets blocked for a day.