Many acute viral infections can cause rashes that can appear on various parts of the body. These rashes are usually light red to pink in color and may fluctuate with fever. Also if you press on the rash it should get white in color. If your son also has fever, cough, and other symptoms of a cold-like illness
then this is probably what is going on, and you can continue to watch it at home. However, if he looks very ill, has very high fever, or is unable to eat or drink then you should take him to the doctor
There are a couple types of rashes that are more concerning for more serious infections or illnesses and should not be ignored.
For example, if your son has a rash that is primarily on the soles of his feet (rather than the tops and shins) this would be very unusual and should get checked out right away.
Also, if the rash appears dark red or purple and does not fade in color very much when you press on it, this is more serious as it could represent an inflammation of the blood vessels and should get checked out right away in the emergency room or by your pediatrician