Is it alright if I tell my best friend's dad that he doesn't care about his son?
Why would you want to tell your best friend's dad that? I need an explanation or I won't believe you.
I can't stand his son because of his behavior.
It's good that you want the best for your friend and for his father to care about him but it isn't appropriate to get involved in another family's personal matters as an outsider. If you think that your friend needs better support that he isn't receiving from his father, you should try to get your friend to start participating in therapy so that he can get the support he needs.
I'm going to tell his father anyway. He's going to fall into depression like most alcoholics do eventually.
It shows you care about his son but the father seems to need help too. Just stay out of it and then let them handle things on their own for everyone's sake. Sometimes things have to hit rock bottom for families to deal with things properly.