We're waiting to talk to his therapist about the latest blow up and aren't sure what we should do.
You don't say whether his therapists have told you that they will also see you in counseling, as part of your son's treatment program. If they are just seeing your son and not you, they might not wish to discuss your son's outbursts toward you, considering it a violation of therapist/patient confidentiality. No therapist that I know counsels one adult family member and then talks about what went on in therapy with another family member, unless the family member in treatment has given the therapist permission to do so. If you haven't done so, I would suggest that you speak to his therapists and ask them what their treatment approach is and whether you will be asked to come in for counseling as well. Many family-centered therapists see all family members as part of their treatment approach.
Even in highly successful therapy, there can be times when it seems like the client's behavior is worse than when he began therapy. Don't expect an immediate change in your son's behavior, attitudes or beliefs. Your understanding and support are needed as he undergoes therapy. But your understanding and support do not need to include taking verbal abuse or any other kind of abuse without a response. Your role is not to be the whipping boy or to feel compelled to agree with everything your son says or does. Just because he is in therapy does not mean that you cannot attempt to have reasonable discussions with him or walk away from him if he is refusing to do anything but yell at you. It also does not mean that you are required to allow him to smoke pot in your home. Beyond the detrimental effects on his physical and mental health, it's illegal. His therapists should know your stand on this.
I wish you and your son much better times ahead. I'm sure that you will do whatever you can to help him.