My 13 month old son had a fever for 3 days and now 2 days after the fever broke has a rash all over. Is this just a reaction to the fever?

Likely roseola. Roseola is a common viral illness of childhood. Often causes high fever with not much else, usually for 3-5 days, then the rash appears and fever disappears. The rash is typically pinkish tiny dots underneath the skin, usually not bothersome to the child. If it is not bothering him, let it be. It will go away in dew days. If having severe symptoms, consult doc...Good luck.
Roseola infantum. A rash that appears as fever is resolving is the classic presentation of roseola infantum, aka exanthem subitum, or (back in the day) sixth disease. Typically the rash consists of pale pink, almond-shaped spots on the trunk and neck which then fade in a few hours to 2 days. If the rash persists it could be something else, and you should see your doctor or a dermatologist.
Viral? I would expect the rash is more caused by the virus or bacteria that caused the fever. My worry is that it could represent the rash that can come with strep and might need to be treated with an oral antibiotic. Strep throat can be present even without the sore throat.