Thanks for posting,
One can never tell the gravity of a head trauma
just be observation alone. Normally children or toddlers knocking their heads often results to just a small bump with nothing serious on the head, but since your son developed a fever and vomiting
some few hours later, I suggest that he gets examined by a doctor soonest. CT scan might be more or less necessary (but could be included). The toddler might be suffering from an intracranial hemorrhage
or subarachnoid hemorrhage
due to the fall.
His presenting symptoms could just be as a result of a small malaise
Hope this info helps you and your son.