A.S.
It could be because that's what he want to do or teeth, like I read below.
At six months he may not be getting enough to eat and needs something at night too.
My husband was working nights in the navy and wasn't even home when my son would wake up so I know what your going through. It might sound mean but make him get up in the middle of the night or wake up early when your son is up for the morning. Leaving you to sleep in a little. It will make a difference no matter how little.
Good luck and sleep as sound as you can.