This sounds like GERD, gastroesphogeal reflux disease. I remember the first time my son did it. Projectile breast milk across the room. For a few years, we found dried milk in all kinds of places. Our pedi weighed him every week, prescribed a couple of meds to help him digest a little faster and most importantly, we were taught to keep him upright for AT LEAST 15 minutes after he ate.
Because this sounds like it started later, I would also ask about a pediatric nutritionist or someone who can help you rule out allergies to what he is getting. He is on formula, right? That stuff has all kinds of things in it, depending upon what brand and kind it is. AND it sits in the stomach longer that breast milk, so if the sphincter between the stomach and esophagus spasms or can't contain the contents, there is more in the stomach to regurgitate. There are other things that this could be as well, so I would make sure that you are keeping records of time of day, before/after nap/sleeping, feeding position, type of bottle, etc.
I called the doc every time we had a GERD episode at the beginning and went down there because he was loosing weight. The meds helped immensely, and after a few months, it just went away on its own. Some babies muscles aren't as strong as others in this area, but they catch up. Keep asking questions, if you don't like what you are hearing, find another doctor. God Bless you! M.