Hello My Son Is 16 Months, He Started A Fever

Hello my son is 16 months, he started a fever 5 days ago, it went away on day 5, he went 35 hours with no medicine, now it's back a day later accompanied by a cough, he's eating and drinking and getting up to play with toys when his fever goes down, how long can I let this up and down go on for before I make a doctors visit, and how high should I let his fever get before giving medicine, I know a fever helps kill the virus, just don't want it to high