Oral thrush is an infection and left untreated can cause several health issues. It can cause fever and spread the infection to the esophagus as well as other organs.
Other than getting yelled at by his dentist, if this is not fixed, your son will suffer cavities, tooth decay, gingivitis and very bad breath. You should take him to a dentist as soon as possible.
Oral thrush can be very painful leading to decreased ability to eat and drink. The biggest concern with oral thrush that is left untreated would be secondary bacterial infections.
