What Causes Vomiting Immediately After Having Breast Milk?

Hi. My son is two and a half weeks old. He's breast fed. He only stays latched on for maybe 7 mins.. then falls asleep. I pull him off to burp him. And he doesn't burp. Just vomits what looks like ALL of what he ate. And it's cloudy and foamy. And he acts like hes in pain when he does this. After I get him calmed down.. He sleeps for about a half an hour then wakes up wanting to eat.. what should I do? I dont want to over feed him. But I dont want him to starve.