My son hit the back of his head

I’m needing some help. My son is 4 yrs old. In pre-k. Today he got home from school and he told me he hit his head at recess. So I checked it and there is this huge lump and it’s very squishy. I’m absolutely scared and so mad that no one informed me anything about this. He told me he told his teacher and that they took him to the school clinic, but why wasn’t I informed. What should I do ? Should I take him

To the e.r . I’m lost in what to do

Honestly picture doesn’t really show how big it is