My son has a spider bite on his upper arm a lil puffy spot and hard underneath, been there for a week. how can i treat this seems to be bothering him?

Hard to say?: If it is a spider bite, perhaps you would want to be seen? Local care of any would usually consists of warm soap and water with gentle washing/scrubbing, then patting dry with a clean cloth or gauze. Application of a topical antibiotic ointment may be needed and then a loose covering usually with a non-stick (telfa) dressing. Observe for pus, redness, pain, heat, streaking up from the wound.

See your doctor: Many times what people think are spider bites actually turn out to be bacterial infections. If it is hard it may have pus collected beneath the skin and require drainage and/or antibiotics. I would recommend that you seek care immediately.