Its no biggie.
I think boys grunt more.
I have a girl and a boy... my son, was a grunter. He grunted whenever he was hungry or whenever he was about to poop, or tired.
He was noisy... grunting a lot. But by his grunting, I knew what he wanted or was going to do ie: poop.
Spitting up is also normal. After feeding... don't lay him down, keep him upright so the liquid does not come back out.
Your son is just a grunter. He is communicating. My son was like that as a baby. He grew out of it as he got older.
My son, was grunting since he was born. In the hospital, I could tell when the Nurse was bringing my son to me, because even as they were walking down the hallway to my room, I could actually HEAR my son GRUNTING all the way down the hall until they got to my room and gave him to me for nursing. The closer he got to my room and then "saw" me, the louder he grunted! The nurses said he's a smart boy!
And no, my son's grunting was not because of reflux.
All the best,
Susan