You have not mentioned any medications he used to take. In that case I can suggest whether it may be drug related or not. In diagnosis of serotonin syndrome clinical examination along with drug interraction study is necessary. A lot of antidepressants, opioids may responsible for this. Supporting therapy and cyproheptadine have role to control this rare syndrome. Again nausea, fever, muscle pain are commonly seen in viral infection. Which subsides generally following treatment of the infection. Symptomatic therapy is also needed. Again there is a chance of developing hypokalemia after several vomiting. Hypokalemia itself a cause of muscle cramps. This can be diagnosed by measuring serum electrolyte level and can be prevented by taking ORS after vomiting. There may also a chance of drug withdrawal symptoms which is commonly seen at sudden withdrawal of antidepressant . So its recommended to visit a doctor if these symptoms persist.