Dramatically changing his personality! How do I stop his addiction which is dramatically changing his personality?

Changing others. I agree with dr. Fowler here -- it is not possible for you to "stop his addiction" if he does not see this as a problem. However, you can take good care of yourself by attending a 12-step program for friends & family of addicted people, and encourage him to attend a 12-step program. Sometimes planned interventions can be helpful -- you need a professional moderator & a plan for immediate care.
Control. Unfortunately, we control others - even when it is painful to watch their self-harm. However, you can encourage him to engage in a 12 step program. Don't know what his addiction is to but there are many programs (food, narcotics, alcohol, sex, gambling etc). Take care.