He'll snap out of it in time. You may want to be his driver in the mean time.
It's his coping mechanism, and he'll snap out it eventually. I would suggest keeping an eye on him when he's been drinking heavily and be his designated driver until he snaps back to reality.
It's not unusual for people to drink heavily after a bad breakup. Just make sure that you're a good friend and supportive and try to help them find other things to do than drink heavily to forget.