Hello Holly,
I am so very sorry to read your story and to hear how badly affected both your son and you are affected by his cannabis use.
i really hope that the rehab and follow up that your son is going through soes help him.
It sounds like you might be in need of support for yourself and you may like to contact us at Icarus Trust. we are a charity that supports people going through what you have and have a rang of services we could offer including listening ears from our very experienced people.
You can contact Icarus Trust on help@icarustrust.org or visit our website http://www.icarustrust.org
All the best to you and your son.