My Son Is 14 And Has The Flu. He Got

My son is 14 and has the flu. He got it from his 8 yr. Old brother 5 days after he began showing symptoms. The 14 year old has a twin brother and seems to be getting better quicker...It has been 4 full days of sickness , going on 5. He does not have a fever, but his feet, hands and head are very cold. Symptoms include: diarrhea, nausea and dizziness. When should I take him to the Dr. If at all?