Hello & welcome
please don't worry as the age of your son is 5 years this may be just an eruption cyst and not require any treatment,
usually at the age of six last permanent teeth start getting erupting, before eruption of tooth such type of cyst formed & get resolved as soon as tooth erupt.

Hello there...if your son is finishing 5 years of age and getting into his 6th year, 1stmolars usually finds its place in the oral cavity....These are permanent set of dentition to erupt initially and lasts through the entire life....Could also be an erupting upper 1st molar...If he is still in his earlier part of 5th year one can suspect carious tooth.....infection from the carious tooth would have spread apically ( below the tooth) resulting in abscess...pus gets filled within it and swells...that further penetrates the bone and presents in the form of a small swelling over the gums....that is soft to firm in consistency...
Clinical evaluation is necessary before confirming the diagnosis... Consult your dental surgeon for clinical evaluation and management.