Your son is vomiting stomach acid. Its weird to see and kind of scary, but that is all it is. If he is vomiting for a really long time and you can not get him any fluids, then get him in as he will dehydrate, reguardless of how happy and playful he is. My now 2 yr old had croup so badly when she was about 18 months, that her lips were turning blue. We rushed into the ER and she ran all over the place even though she was barely breathing. They kept asking me what was wrong, and I kept saying she wasnt breathing right. Well when they finally did the oxygen level test it did show her levels were way down, and she spent the night in the hospital. All the nurses thought it was funny how much spunk she still had even though she was barely breathing. So as you can see, every child handles being sick differently. I once got into trouble with my son when he was 5 months old. He has an ear infection and by the time I took him in, it was very bad and they asked me why I waited so long. I said How was I suppose to know he was sick if he was playing, moving all over the place and eating like a pig. It did shut up the doctor after that!! lol Remember YOU know your child best, so dont be afraid to push issues with them and if they ignore you, find a new doctor. If I hadnt pushed, my now 2 yr old might not be with me today when I was pregnant with her. Good Luck!