thank you for your query.
thank you for the detailed history.
yes it seems like your son must have sustained a fall.as infection and fever
doesnt have that effect.
It is likely that this is just a cyst
under the skin. Cysts can form anywhere on the body under the skin, and they are almost always benign and nothing to worry about.
If you are interested in having this bump evaluated further, you should see a dermatologist
or a surgeon. If your primary care
doctor performs office procedures, that would be another option. They will help you decide whether or not to have the bump removed or whether to leave it in place for now.
wishing you good health