Do you think it's right if I tell my partners family about his substance abuse/addiction?
Why would you reveal that to his parents?
His relatives need to know that their son may potentially be slowly killing himself.
It's a good thing of you trying to deal with your partner's addiction and substance abuse, but I think your reason of telling his family has malice and you have an ulterior motive of doing so. How about you talk with your partner first, and see if he is comfortable telling a private matter to his family members.
I'll talk to him once I am ready to think about this more.
You should work on this with him alone. Ask him if he wants his family to know and give him a chance to seek help on his own first.