Hello and thanks for the query
after head injury
is a matter of concern but not panic. The cause of fever may be infection in the wound or collection of fluid or blood at the injury site. As the child has loss of consciousness
you must have done CT head in the hospital. IF not done it now.
IF CT head is normal than need not to worry and you have to take an effective antibiotic to get control over the fever and possible infection. Meanwhile take Ibuprofen
to reduce fever and pain.
Consult your treating doctor of fever aggravates.