Possessiveness to this degree would be considered toxic to the health and well being of the male family member. Since it's effected his life in such a negative way, I would definitely call this relationship toxic.

What you can do is gently bring it up to the male family member. You can reflect back to him your observations of his changed behaviors and how this relationship seems to be effecting him negatively. You can express your concern for his health and well being, and you can point out the possessiveness and subsequent isolation as not being healthy.

I would personally say something since it is a family member you care for. If I were in his shoes, I would want my loved ones to be honest with me.