I dont think you should be concerned.Its not putting his life at risk. People all have different sleeping habits. My boyfriend sleeps with 3 fingers in his mouth,and doesnt suffocate hisself. So I dont think hiccups is something to be concerned about . Maybe you should make him drink something hot before he sleeps?
Has he done this all his life?

No he has not had this occur until about a month ago. I wondered if it was related to not breathing correctly at night.

Maybe you should check with the doctor. Or if you have a stethescope put it on his back and make him take long deep breaths if its something to do with his breathing you will hear it through the stethescope. If your worried id check with the doctor.
Good luck.