Mu son who is 10 was seen yesterday by our family doc. For what appeared to be spider bite on his left lower calf (outer side). It was swollen and red. He was put on sulfamethoxasole. The area had to be lanced and drained. It is being tested for MSRA.

It's ok. Yes , could be spider bite and then got super infected . The doctor was right to culture for MRSA. .
Abscess. While you gave us good information, you didn't ask a question. What can we help you with?
Yes. spiders can carry MSRA and sulfamethoxizole covers MSRA.
Not spider. Sounds like ordinary boil. Drainage helps. Sulfa probably of no benefit. Spider bites are very different. Your DR should read-up on these subjects.