Hello dear, Thanks for being on healthcaremagic.com.If you would have presented to me as an outpatient,I would have made the following clinical explanation- Fever,rash,headache and myalgias(leg pains)-this syndrome fits into a viral syndrome and so,I believe that your son is suffering from a viral infection.Please discuss with your physician,that after history and physical examination.what is his likely diagnosis.Although,most of the viral illnesses are self limiting,some of them may be treated with antiviral drugs like acyclovir.I hope that this will improve soon.He needs adequate hydration,antipyretics and adequate rest.Hope it helps.