What Causes Vomiting At Night?

Hi My Son is 16 months old. He has been Vomiting ONLY at night for 12 days now. He is perfictly fine during the day. Laughing, playing, eating although it is very limited and drinking alot during the day. He is Very pale. We took him to Emerg on Friday night where they thought maybe it was a Brain tumor. They sadated him and gave him a CT which came back clear. They wanted a urine sample but he wouldnt go so they tried to give him a Cathader about 5 times and couldnt get anything. SO they sent us home with a bag to try to get some. I was to take it in as soon as he went. Well then he didnt go until the next night. So he went approx 22hrs without peeing. So I took him back in. They tested his urine which came pack clear of a bladder infection but had some proten in it so they did blood work to check his kidneys. That came back clear so they sent is home AGAIN. I went to my Family dr yesterday who was at a lose but did weigh him and he has lost 3lbs which is alot in a 16 month old ( he was just weighed there a month ago at his 15 month apt). So she decided to send us to a pediatrician today who before he even looked at him was like... Its a virus because he was happy, playing, talking, walking. Which is what I have been saying the whole time. That hes FINE during the day. At the begining he was vomiting about 1-2 times a night now we r up to 7-8 times a night, he is now haveing stools that r like water which wasnt like that at the beginging and is now only peeing about once or twice in a 24hr period. They dont think he is dehydrated because he is still drinking. He has a twin sister who shares everything with him including the same room and she is perfictly fine. Im just wondering what I should do. Some people are telling me that I should take him back to Emerg and tell them Im not leaving I have an answer. Im concerned because it just keeps getting worse NOT better and the pediatrician we seen today wants to wait it out another 2 weeks. I think thats too long. Im at a lose as to what to do anymore. Do I trust what they r telling me and leave it alone OR do I go in there kicking and screaming until I get an answer?