My son did this and it always looks like a lot more than it is. If he's gaining weight, he's obviously getting nourishment. You'll hear a lot of things about reflux, telling you to do GI testing and getting meds. You'll also hear a lot about so-called allergies (to soy, to dairy). If you have watched the news recently about the peanut allergies that have been misdiagnoses and improperly treated, you heard that they have discovered that testing positive for an allergy doesn't necessarily mean that the child really has that allergy! So all kinds of nutritional programs that have kids avoiding certain things are now called into question. If your son is spitting up once a day and not constantly, it's hard to believe he has a true allergy or major medical problem.
You can try burping your baby more frequently during feedings. Another thing to recognize is that all formulas are not the same - even if they all say "soy" for example, what type of soy is it? Whole bean? How is it processed? What else is in it? You can't really tell from the label where the product is manufactured and you certainly can't tell what process is used, where the soy comes from, etc. Since soy is nature's protein and has been used by the world for 5000 years, it's highly unlikely that your child really is allergic to it! More likely, something else that's in there.
You could use Now for Kids from Reliv. It's used worldwide feeding over 40,000 kids per day - for many, it's the only meal they get. It's made here in the US and uses top quality nutrients. It's a fantastic supplement that clears up a lot of those digestive issues by giving kids what they need in the right balance and in a highly absorbable form. It was formulated by the same scientist who was on the team that developed ProSoBee and Enfamil formulas. He also was the consultant for Weight Watcher's foods so he is highly regarded as an expert on nutrition, and particularly infant nutrition. The product is safe and carries no warning labels. I would think you might want to start there rather than putting drugs into one so little! It will keep you from worrying about him getting enough nutrition even if he spits up for a while yet until he outgrows it. Let me know if you want more info - I know so many moms who use it and their pediatricians say it's perfectly fine.