Boyfriend drinking problem?

when we first started dating he would drink a lot. Hard stuff, which is something he has been dong for years. He would drink a couple times a week but his behavior remained the same. Now that we are more serious and much closer he drinks much less but it seems to have a much more emotional effect on him. He now becomes erratic and has emotional outbursts that alwaysend up in him hiding his face the next day or apologizing for what happened. It always ends with him storming off like a child. He has niot control over his emotions when he drinks now. Any clue why his behavior would change like this?