My Five Year Old Got Hit In The Head With

My five year old got hit in the head with a bike pedal (the bike was upside down and he was playing with it). At the time we had no idea he had been injured because he is autistic with very high pain tolerance. I just laid him down for bed and noticed he has an inward bump, a dent in the side of his forehead that is brown and squishy. I am extremely alarmed and researching made it worse. How bad is it? It feels like the left side of the smallish dent is raised and then goes into the squishy, brown dent. Please help!