My 5 year old has some small blisters inside his mouth and outside! his gums are also swollen and he complains that his teeth hurt when he brushes wha?

Viral, maybe herpes: There are a few viruses that can do this, but the most common is an initial infectioin with herpes simplex virus. You need to take him to the doctor ASAP, primarily in case it's herpes, often acquired at his age from schoolmates or playmates. Herpes can be very effectively treated, and the sooner he is diagosed, the easier the diagnosis and more effective the threatment. Don't delay!