Like you, most parents are concerned when their child is sick and want to do whatever they can to make sure they get better. Before you take your child to see the doctor, you should note that children can get as many as six to 12 colds and flus a year and that symptoms usually last up to 10 days.
In this particular case, your son has had a cough for three days, which can be very irritating and hard on a young child. Keep in mind though that most coughs do not require antibiotic treatment. For now, try to soothe your son's cough with an over-the-counter cough medicine. Your pharmacist will be able to help you choose the best cough medicine.
Watch your son's health carefully for the next week or so. If he develops a persistent high fever or he suddenly gets worse, you should take him to see a doctor. In general, children should see a doctor if they appear to be very ill, suddenly get worse or if cold and flu symptoms last longer than two weeks.