Hello, I'd like to help you with your question.
This is a complex question so let's take the issues one at a time:
In terms of the alcoholism, an ultimatum is usually very helpful. But when you factor in PTSD, which is sounds like he has, then you may have symptoms that are linked and that can complicate how to approach him about getting help. Alcoholism can begin in a person as a way to cope with PTSD. And when that occurs, an ultimatum can affect mental health aspect of the person as well, making them feel upset and threatened. In this case however, treatment for alcoholism is vital so the PTSD can be addressed. A gentler approach might be an option such as using part of the ultimatum such telling him that he needs a proper assessment and a treatment protocol and pursuing that first.
The type of counseling your brother in law is receiving is not appropriate to the symptoms and diagnosis he has. It is in essence like seeing a chiropractor for treating cancer. Your brother in law needs to be seeing a fully trained mental health therapist. Here is a link to help you find one:
I suspect that your brother in law might have PTSD which is not treated exclusively by CBT and definitely not by positive psychology. Instead, a treatment method that is designed to deal with PTSD such as trauma based counseling tends to work best. Search for a counselor that specializes or has training in PTSD.
As a part of therapy, you can suggest that one person from your family is able to access your brother in law's therapy process. Any more than that and he may feel too overwhelmed and withdrawal and refuse treatment. And anytime anyone has something they want to add
, family and friends are free to contact the therapist. However, without a release of information, the therapist cannot talk to them. But at least the therapist would receive the information.
Regarding your last question, that is probably not the best idea. If you cut him off, then he may feel rejected and go downhill in terms of his symptoms. Make a simple request to him that he continue with treatment and be supportive. And as long as you let him know you are there and supportive of him seeking help, he will be encouraged.
I hope this helped you,