I have a friend whose kids have very weak cardiac sphincters. That's the circular muscle at the top of your stomach that squeezes shut to keep things from coming back up. It opens up again to let burps out, to throw up, to let food in, etc.
It really sounds to me like this could be a problem for your little guy. If he lays down and food just comes out, that would trigger vomiting, I would think.
I'm not a doctor, but I did extensive studies in human anatomy and biology. I also have three kids and babysat regularly. This is NOT normal.
I babysat a boy who would throw up when he didn't get his way. When he threw up, he got all sorts of attention. His mom really screwed that kid up, in my opinion. She'd tell him no, you can't have something, so he'd cry really hard, and then she'd pick him up, say she was sorry, hug and kiss him, then give him the thing he wanted. When he threw up, she gave him more and more and more. My sister and I, just from babysitting him frequently, stopped that behavior by making him clean up his own barf (he was only 2) and not giving in to him.
You don't sound like that kind of mom.
And sliding down the weight charts is NOT okay. My husband is very tall and skinny (6'3" and 180 lbs) and none of my kids have changed their percentiles pretty much since they were 1 month old. I have one who is 75% on height and 50% on weight, the second is huge at above 97% on height and 85% on weight, and our skinniest is 90% on height and 50% on weight.
Frankly, I would change pediatricians. If you are worried about your kid, your ped should be, too. I have another story to share about that.
My second baby was the huge one. I breastfed all my babies. When he was 3 1/2 months old, I started taking the mini pill. Baby Z stopped flying through his clothing sizes like he had been (he was in 9 month clothes at the time) and cried a lot. When I took him to his 4 month check up, he'd dropped in his weight percentile. But because he was usually in the 85th %, and he dropped to 50%, I was really worried. But the doctor wasn't, because he was still 50%! I didn't feel right about it, so I followed my mother's intuition. I stopped the pill and suddenly, DS shot out of his clothes and started sleeping more and crying less. He'd just been hungry! He popped right back to the 85% and has stayed there for 5 more years.
Kids should not be changing their percentiles. Mine have stayed constant. I guess the story would be different if your other kids didn't stay constant.
I have a pediatrician now that defers to me most of the time because I have three kids that turned out great. He asks me what I have done with the other boys, if they talked early, etc. He listens when I have concerns and draws medical diagrams to explain things to me. I think everyone deserves a pediatrician like mine, although I think he's a rare gem.