It may just be a mosquito bite, because after a while the hole where the skin was punctured scabs over and makes a black dot.
It could be a Staph infection that they were referring to. I've had one before and the exact same symptoms to go along with it, except for the high fever. But since your son is an infant high fever is common. His immune system is weaker and just developing and really being exposed to all these organisms(not a bad thing), but with that being said his immune system will work a lil harder and be a lil more extreme due to this, hence the fever. The big thing to worry about with staph infections and any kind of bacterial infection is that it does not become septic(bacteria in the blood). Hopefully they gave you some antibiotics; that should be all he needs to get the infection under control. Hope this helps!
This is due to infection because insect that bit your son leave poison in your skin that has caused infection to your skin. Using anti-biotic can help you to get rid of hard spot to your body.
I don't know for sure, but I got bit by something where the same thing happened. After a few hours though, it spread and ended up being a huge wound that was almost eating away at my skin. I went to the hospital a couple days later because it kept getting worse, so if this is the same it was for you, they guessed that it was a brown recluse or hobo spider.