how long does it take till cannabis is clear from any effects on the body and mind, especially with concentration and the abilit

My 15 year old son starts back to school Monday, and he will be in a challenging situation this year. He is ADHD and has impulse control issues and oppositional issues with any authority figure. Over summer he has openly decided to smoke cannabis, I am sure it is daily! Very frustrating!!!! He odes want to make this year a success, and I am hoping that he will get his body and mind cleansed this week in preparation for Monday.