I would wait 24 hours before heading to the doctors if he has no symptoms other than vomiting (ie. no rash, no fever, no pain on urination...). Keep giving him small, frequent sips of water and small BF's, very plain food if he is hungry. Keep him quiet today at home. The fact that he is happy to still drink is a good sign. You will know if he is dehydrated as he will not wee very much (and it will be dark and smelly), his lips and tongue will be dry and he will be very listless. If he keeps having sips of water he should be fine, even if it comes up pretty quickly. I assume your DS is usually fit and well and doesn't suffer any chronic illnesses, such as diabetes etc?

A doctor will not do anything for your DS at this stage and if it is a bug, taking him to a busy doctor's surgery will just spread those germs out into the community. If you are very concerned, you could give the practice a call, do they have a nurse you could speak to?

Having said all that, you are the best judge of you DS's health and if you think he needs a doctor you should take him. I hope he feels better soon!