D.,
I am a stay home mom of 3 little ones myself. My oldest is 5, then there's my 3 1/2 yr old and my little guy who's 18 months.
You are right to be concerned. Babies should not be spitting up after they eat, and as one mom said "I asked the doctor about it and she said that he might have acid reflux. Which is very usual now for babies." There is so much wrong with this I don't know where to begin. Babies are formula fed at such an alarming rate over breast milk today than ever before. Breast milk is a living food. Formula is not. Many of these formulas are chemical in nature, and the vitamins they contain are not in their natural state. And bottle feeding (regardless of contents) is synonymous with overeating, since the bottle is designed to almost force the feeding reflex by placing a slow but constant drip into the babies mouth, and down into their throat. Most babies can't help but finish all of what's offered to them in a bottle unless they are constantly burped throughout the feeding, to be able to recognize their bodies natural signals that they are full. Who doesn't get an upset tummy after a nice big Thanksgiving dinner? Remember that their stomachs are only as big as their fists.
It is entirely possible that your son is overeating. You also mentioned that you supplement your breastfeeding with formula. Without knowing the specifics of your supplementation I can't comment as to how that might be affecting his nursing sessions as well. I'd be happy to talk some more about it with you if you PM me.
I would suggest for now to eliminate as many of the formula feedings as you can (and/or) Reduce whatever you're feeding him at each formula feeding in half, keeping him upright during his feedings. All babies including breastfed babies need a good burping after each feed. If he goes down for a nap after any of his meals, try keeping him awake and upright for at least 15 minutes before putting him down.
Best wishes that your little guy finds relief and that you find the answers you need.
Blessings,
Rolinda
Wife, Mother, Friend