Hello and welcome to HCM.
Sorry to know about your nephew.
The clinical scenario you described looks like he must be having viral
haemorrhagic fever like
dengue,chikungunya transmitred by mosquito.In
lyme disease,there is history of
tick bite and rash is limited to the bite site in most of the cases.Though we cannot completely rule it out.
Basic and specific blood tests will rule out the disease.
In chikungunya,illness is generally limited to 7-10 days.
The prognosis of dengue depend upon the clinical and blood parametres of patient.May require vigourous in patient follow up in hospital with iv fluids,platelet transfusion in case of haemorrhagic complications(
renal failure as one of complication causing decreased urination).
If detected with lyme disease,course of antibiotics will generally help.
Hope this information helps.
Wish him speedy recovery.
Thanks