your son needs to be re-examined by doctor.
Viral infection could be the diagnosis. But, they usually lasts 5-7 days. Malaise, headache, anorexia (loss of appetite) , vomiting, diarrhea
, weakness are general symptoms of viral infection. But, 12 lbs weight loss
is something unusual. History of head injury
may (or may not be) relevant for his present illness. Clinical examination is necessary to reach diagnosis.
Yes, blood tests may be necessary. But get the clinical examinations be done first. Clinical examination findings can point out the affected body system and laboratory reports are done accordingly to support it.
Please visit your pediatrician