Negative RAST and Skin Test but showing a reaction

Please help me understand what is happening with my 18 month old son.  Around his first birthday he had exposure to egg white and had an allergic reaction (hives all over his body and dirreah).  My doctor recommended we see a pediatric allergist and prescribed an epi-pen for the time being.  Since this appointment with my doctor I have noticed that my son reacts to egg that I eat through my breastmilk and has had allergic reactions to even traces of egg (we have not intentionally given him egg, but there has been some accidental occaisions where it was a trace ingredient in something he ate).  

Just last week we finally saw his allergist (6 months after we were refferred), he had a skin test and it proved negative.  However, after we left the doctor's office I noticed that he had small hives all over his body.  These hives lasted nearly a week.  We took him for a RAST test and it came back negative as well.  The doctor told us that it is unlikely that he is allergic to egg, but we are to avoid it until he has an in-office food challenge in 6 months.  

Could someone please explain to me what is happening with my son?  I am very confused at the results of his test.  Why would his tests come back negative when he has hives all over his body from the scratch test??  I am very confused :(

[Tammy2009]

I had a very weak RAST response to my allergies and a very strong skin reaction (plus normal allergy response in normal life).

I would wait for the food challenge and go from there.