HI, thanks for using healthcare magic It is likely the flu as you stated. This would cause headaches, fever with chills, muscle and joint pain, sore throat. Dehydration would not be normally associated with fever,aches and pains or sore throat (throat may feel dry but not usually painful). Most cases of the flu can be managed at home but if the symptoms are severe then he may need to be assessed by his doctor. Treatment would generally consist of : (A)fluids , rest (B) topical meds such diflam, covonia throat spray, chloroseptic throat spray- may help with the sore throat (C)may need to combine different types of pain killers such as tylenol and advil If the pain is not resolving,then he should see his doctor (D)over the counter cough and cold meds would help with other cough and cold medication I hope this helps