Hello Good Evening,
I think you should not worry a lot, the child is normal and that was not a serious
injury due to rubbing of your chin to his head.
Your chin most probably hit at
anterior fontanelle region of his head, and you should aware that why you feel low and soft there.
The anterior fontanelle is placed at the junction of the
sagittal suture, coronal suture, and frontal suture; it is lozenge-shaped, and measures about 4 cm in its antero-posterior and 2.5 cm in its transverse diameter.
The fontanelle allows the skull to deform during birth to ease its passage through the birth canal and for expansion of the brain after birth.
This anterior fontanelle is not completely closed until about the middle of the second year.
So this was present since birth, but you noticed it after that event, so be relax, if nothing deterioration occurs in his health.
Regards.