Hi S.! Congrats on your newborn. These little guys are always full of surprises! :)
I have 2 boys. My oldest is 4 and my youngest is 20 months. BOTH of my boys spit up. My oldest was the king of spit up. He really did a lot. My younger did as well, but not as much as my big guy.
You didn't say if you are nursing him or if he is formula fed. I nursed both my boys. My ped checked them out and she was very calm about the spit up issue. She told me that not only are babies digestive systems immature, but the muscle that is supposed to hold food down when they burp (boy, I'm just using all the proper medical terms, aren't I...ha) isn't fully developed yet either. She said it usually strengthens between 6-9 months which is exactly what happened with both my kids.
The other thing...again I don't know how your baby is being fed...with my first I was nursing him CONSTANTLY...he was not on any schedule and any little peep he made I fed him. He was the CHUBBIEST baby ever and I feel now that I definitely over fed him. He needed to be fed on a schedule. My youngest I fed on a schedule. He still spit up but not nearly as much. Anyway...that's just my experience. Next time you go to the doctor just ask.
All the best to you and your family. Keep the questions coming! :) We're all here for ya.