Caput succedaneum or simply CAPUT. It is a swelling of the scalp as a result of prolonged pressure on it during labour. It will resolve spontaneously and it’s nothing to worry about .

Just apply ice packs intermittently and you can give paracetamol every 6 hours for the next 2 - 3 days.
Do not rub it and don't use hot water.

Did he fall down at some point? Try to do your investigations. All the best