Hi Jane,
You can see from the responses that your son's problems can range from nothing to something that's potentially serious. This is why you need to take him in for a pediatric evaluation, and not go by just the advice on this site. Yes, it could be allergies, reflux, or even a pyloric stenosis, but it could also be a simple tummy bug or something else. Just because you haven't had him out of the house doesn't mean that he hasn't picked something up from someone who's come to visit. Also, is he spitting up or actually vomiting? The distinction is important and you need to give the right information to the doctor. Baby's may spit up up a tablespoon or two at a time, which can seem like a lot but isn't really. However, if your son is vomiting can look more like a waterfall cascading from his mouth. In some cases, infants (especially with a pyloric stenosis) will projectile vomit and can literally hit a wall 3 feet away.
I think the most important concern is whether or not he's getting dehydrated from lack of feeding. If he seems listless or the fontanel seems shrunken, he probably is getting dehydrated and you should see a doctor ASAP.