Don't be forceful about it either. I can only speak for alcohol abuse, but I imagine it would work the same with this. They usually won't change unless they want to change. Either that or a life changing experience. I unfortunately had to endure the latter of the two. A blessing no doubt, but never fun experiencing it. Many people tried to talk to me in the past and I didn't want to listen. So be prepared for that. It's not fun for him any more than it is for you to watch it happening. Stick with him. Don't give up on him if you truly care. I had 3 people that never gave up on me and they were my parents and now wife. I can go on but it would shift more towards alcohol rather than diet and exercise and I'm not sure if they can be talked about the same.

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