Its better that you try Paracetamol
first- three or four doses in a day. If its simple soft tissue injury
, paracetamol will help for the pain and he will get okay in a day or two.
If he develops vomiting or headache persists its better to have him seen by a Pediatrician
Usually danger signs in any head injury are loss of consciousness
, recurrent forceful vomiting, ear or nose bleed, watery fluid leaking through the nose, fits etc. These things, if seen, warrants an urgent consultation with your doctor.
Hope my answer helps you.
Wish your son good health.