This Young Lad I Live With. Disturbing Issues Watch
I think your best bet is to get in touch with social services. It sounds like you're not going to get anywhere talking to the dad if he's aggressive and has such alcohol problems. Obviously it'd be really difficult and painful for the kids if they were taken into care but hopefully if they went to a foster family for a while they'd have some security and a happy environment and maybe their dad can get the help he needs. As it is they're having to deal with neglect and abuse and there's no good for anyone in the situation.
Seeing the boy's headteacher, as someone suggested above, is a good idea. If you don't want to go to social services straight away, you could go to the head to talk things through and find out if they know anything about the situation. They probably have some idea and will probably contact social services themselves, because it's their job to look out for things like this.
You're not grassing the dad up, you're looking out for the kids. Good luck with it all, let us know how it goes and I hope it all turns out ok for them.
That father is scum, and deserves to be locked up somewhere.