That soft bump is due to the resoving contusion
occured after head injury
. As it is decreasing in size it will subside with time and the sourrounding bony feel will be there because of clacification of organized hematoma
and nothing to worry. As you told he had hit again so observe for irritability, persistent crying, vomitings, behavior and for any abnormal movements. If youy find any abnormality the report immediately to your Pediatrician
or emergency department.