Should I stay or should I go? Can a leopard change his spots?

My Fiance and I have been together for 3 years (been best friends since we were 11)

Sometimes when he's had alot to drink he becomes very aggressive for no reason and starts bullying and teasing etc. Although when he's sober hes a loving and caring person.

When I was child my mum had the same type of partner and I swore I would never be silly enough to be in that type of relationship.

Last night my partner got very drunk and aggressive to the point where it really frightened me. I took the dog and stayed at a friends house.
He rang tismorning appologising and wanting me to come home.

I told him I wanted to end our relationship, because everytime he says it wont happen again and it does. He now says he will stop drinking all together for the rest of our lives to keep me - but due to both of our families lifestyles I just dont see this happening. Im meant to marry him in 4 months but im having serious doubts...

I love hime.. but

Is it worth staying with him to see if he can stay off the drink?
Or is the grass greener in another paddock?