Dear J.,
You didn't mention what your son was hospitalized for. Did you ever receive a diagnosis for his illness? What tests were done? What were the results? All of this information is very important for working with your son's pediatrician to figure out what is happening now.
The key message, before I go any further is: GO BACK TO THE DOCTOR! Don't be afraid to be a pest, if you have to be. You clearly have a sick infant, and the cause needs to be determined and treated.
If your son is refusing food, the likely causes (not diseases -- just general causes) are that he is in pain, he is nauseated, or he is satiated (full). Any one of these causes can stem from a myriad of different possibilities.
Many of the moms have already given you good suggestions on what to speak to the doctor about. GERD is actually pretty common in infants, but it's usually associated more with colic-type of symptoms. Nausea, as from any number of stomach and/or intestinal viral or bacterial infections, can cause vomiting and refusal of food.
One mom mentioned pyloric stenosis, and this came to my mind almost immediately, too. Early typical symptoms include projectile vomiting (my son was able to hit the wall at high force from 3 feet away!), or vomiting towards the end of a full nursing or bottle -- sometimes as the baby is nursing. It happens because the pylorus (the juncture between the stomach and small intestine that controls the flow of food) swells, stiffens and eventually closes. When the stomach fills up and the food has no place to go, the upper "valve" (called the cardiac sphincter) between the stomach and esophagus spasm and the baby vomits. For more complete information, go to
http://health.yahoo.com/digestive-symptoms/pyloric-stenos...
Also, because the stomach remains distended, the vagus nerve (which tells us if we're full or hungry) remains stretched. This sends signals to the brain that the person's not hungry. If this is the case with your son, that is why he is refusing food. Mixing cereal into the breast milk or formula will only make things worse if this is what's going on.
Many doctors think of pyloric stenosis as a rare condition as it only occurs in 1 -to 4 out of 1000 live births (.1 to .4%), so they may not consider it early on in the diagnostic process. But it's common enough to warrant investigation, which is very simple: the baby is fed a "formula" of barium and the radiologist takes X-rays over the course of about 1/2 hour to see how the barium passes through the stomach. The solution is surgery to weaken the pylorus muscle so it can open appropriately.
Having to wait for your infant to come out of surgery is VERY scary (our son's surgery was done at 5 wks of age), but it's a pretty simple surgery with very low risks.
Good luck, J.. I'm sure if you persist with the doctor, your son will get the right treatment for whatever it turns out is causeing the problems.
R.