It's not really 'vomitting' at this age - it's a possetting action. Basically the valve at the top of the stomach is not very strong and can easily let go of some of the stomach contents (so if bub needs to burp, as Kate has said, or even if you just jiggle them a bit more than what it can take). It always looks like more than what it actually is! If he is still hungry, by all means, keep feeding him. If you think your let down is fast, try feeding him reclining backwards a bit or have a look at some of the other threads on here that deal with oversupply/fast letdown.

His tummy valve will strengthen in time. Otherwise, plenty of cloth nappies to catch the excess! My DD didn't posset that much, but she always had a bit come up. She stoopped at about 8 mths.