What could this be?!? HELLPPP

So I went to pick my son up yesterday from daycare and noticed what looked to be a bug bite on the inside of his leg. I brushed it off thinking it was a mosquito bite. Well later that day it spread so I drew a circle around it. This morning when I checked it I noticed it seem to have gone down but there was still a decent size red mark. I told daycare to keep an eye on it and when I checked it this afternoon it was bruised. I don’t know if I should take him in. Can the doctor treat this?? First and second pic is yesterday evening and third one

is the pic from today.