Hello again and apologies for the delay in coming back to you.
I would need to agree with the therapist that his parents could not possibly pressurize him more in an effort to go to school. This would be most likely counter productive and could have devastating effects that are difficult to predict. In the same way he could not be made to go to therapy as this would not work. I do understand your concerns and I think that you also have a point there, that certain structures would need to be in place. If for example refuses to take on the responsibilities for his age then his parents would need to make him understand that he needs to choose how he is going to live his life and support himself. They could discuss this with him and tell him that if he refuses school then he would need for example to work and make his living. Of course all of his financial support would need to stop apart from the basics such as food and accommodation, which I would assume are already provided since he lives at home. If they are not giving him money then he may be stealing the money from them for drugs without them knowing. This needs to be addressed. Basically, he needs to be told that he can make his own decisions if he wants to but this has certain implications such as that he needs to make his own living. there should also be strict rules about not doing drugs at home.
It sounds like his parents are struggling with boundaries and I think that they are doing the right thing to see a therapist. he/she may be able to help them manage the situation and hep them to present to their son with a consistent structure and plan/strategies that they will both agree on and be on board with. This is a difficult situation and they need all the help and support they need.
Hope that helps
Please feel free to share any feedback on this. I wish you all the best