What is up with my son?!!

O.K., so my son is almost 4 months old and was in the 95th percentile for weight when he was born (9lb 4oz). Now he is down to around the 25th percentile due to the fact that the kid will not eat!! Where did this kid come from? In my family, we LOVE to eat...as you can tell by looking at us. I never had this problem from my girls. They were always happy to eat whenever given the opportunity to do so. He's gone the past 3 weeks now without gaining ANY weight at all. The dr's are running some tests on him to try and figure out a cause. I'm a bit worried because he said that if he hasn't gained weight by next Wednesday, he may have to be admitted to the hospital for further testing. I just can't figure it out! I just hope and pray that he starts eating good soon. I don't want him to have to stay at the hospital!! Well, that's all I can think about right now, so nothing else to blog about.