My Husband Has Had A Fever On And Off For

My husband has had a fever on and off for 3 days. Fever goes up to around 100 then back down to normal. Feels achy, some coughing. Feels very tired, has been sleeping a lot. No nausea or upset tummy. He is 54, has high blood pressure, no other known conditions. He tends to be dehydrated because he never gets enough fluids. Has sinus infections frequently, says this doesn't feel like sinus issue, no sinus pain or pressure.