My bf has a spider bite on the back of his leg thats about the size of a quarter it was red at first but now it is dark redish purple is this serious?

Possible necrosis: This sounds like the bite may be becoming necrotic. A brown recluse bite frequently acts like this. Your bf should see a physician as he may need to have this area debrided and or may require antibiotics.

See a physician : He may be developing an abscess. You should consult a physician and get treated with antibiotics and incision and drainage if necessary.

Potentially: sounds like a brown recluse spider bite that is progressing. see a doctor and get a tetanus shot if not up to date.