There are a few reasons why bleeding can occur in a child's mouth at this age, it could simply be a new tooth growing (erupting) or it could be inflammation of the gum (this can happen if his teeth are not brushed daily) or it could be a herpes simplex virus infection among other things.

You need to take your child to the Paediatric Dentist for proper check and treatment. There are many things that can cause swollen or bleeding gums. Either the child's teeth are not properly cleaned or the child has a bacterial or viral infection e.g Herpetic gingivostomatitis, don't mind the grammar. Whatever the case may be he needs to have a proper check and treated accordingly