I think my best friend is changing for the worse
Do you think he's becoming a worse person?
Yes, I don't recognize him anymore.
I'm sorry your best friend is being different these days, but everyone changes. You can either leave or stay with him and express your thoughts on what is happening to him. It might not be fair for him for you to just leave because he may just be going through bad times.
He needs to change his attitude. I can't talk to him until he changes.
That's understandable, I know you're upset with him. I hope he'll listen to you and that he'll change for the better so you can continue being friends.