Had the same problem with my little dude. From 1-3 weeks old he would throw up one whole feeding a day. He's seven weeks now. Haven't had a problem since three weeks. I wouldn't worry about it unless it's with every feeding. Sometimes the valve between the stomach and the esophagus hasn't fully developed by that age and the milk/formula will just come right back up. Try keeping his head elevated above his tummy for about 20 minutes after a feeding. Don't be afraid to ask your doctor or pediatrician though. :) Good luck with your little one! :)

it's pretty normal and no cause for concern unless he is just not keeping anything down at all