11 y/o son has enlarged liver, nausea, stomach pain

[Jenbug]

My 11 y/o son has had these symptoms for the past 3-4 weeks:  occasional headache, stomach pain, nausea, enlarged liver and decreased appetite.  He has only actually vomited about 2 times and had diarrhea once or twice.  His pediatrician ordered blood tests to check liver functioning and white blood cell count.  He also ordered a CT scan.  Everything came back within normal limits.  His activity level is normal and active.  He did sleep a lot more this weekend when he started vomiting for the first time.  

What else should we look into and what are the possibilities of his diagnosis?

Thanks  

[Jenbug]

There are no food intolerance issues in our family.  His diet has not changed it is always pretty much the same- nothing unusual.  No rashes or fever.  He has only thrown up 2 times and it was just his food.  The 2nd time he vomited stomach juices (yellow).  No significant change in bowel habits.  And the blood test did test his liver functioning but I am not sure if that includes pancreatic enzyme levels.  All of this did start after we got back from a cruise to the Bahamas.  

[CalGal]

Are there any food intolerance issues in the family? Have you tried tracking what he's eaten and the symptoms - a food diary? Any rashes? What is he bringing up when he throws up? Any changes in bowel habits? Did the blood tests include pancreatic enzyme levels?