I had an over active let down which basically too much milk was released and my daughter took toouch in too quickly and it made her vomit. I found that pumping for about 5-10 minutes before a feeding helped reduce how quickly my milk got to her and the vomiting stopped
You may also want to look up overactive letdown. That happened to me. I had an oversupply of milk and my baby would choke while feeding.
Is he breastfeeding or having the breast milk from a bottle? If the latter, he may be drinking too fast. If from the breast it could be reflux but it could also be an allergy to something in your milk. Also is he throwing up a lot (projectile) or just a little bit. If.it's just a little bit it could just be normal spit up. Unfortunately if it is reflux most doctors won't do anything while baby is young or if he's still gaining weight. I suspected at 6 weeks that my daughter had reflux and my doctor didn't prescribe any meds until she was 7 months old. :(
He could have bad reflux. Are you keeping him in an upright position for at least half hour after feeding?