In that area it might be hard to tell, the swelling wouldn't make it easier. I'm going with everyone and say it's an infection. A puncture wound needs to heal from the inside out. If he got bit and it scabed over it will just fill with puss.
He could have got bit, bug bite, bee sting, scratched or even scratched himself.
I'm hoping it's something that simple and they will just have to drain it and give him antibiotics.

Goodluck.