What kind of spider bite might this be?

My son believes that he was bitten by a spider that dropped down from a tree onto his neck and then bit repeatedly, going under his shirt, down his shoulder and arm. I don’t think he saw it. He felt it.

The bites produced large welts with blistery centers, very itchy.

The next day he felt as though he’d been “beat up.” He has spent most of the past few days sleeping and not much else. He looks sick but doesn’t seem to have a fever.

I’d like him to see a doctor, but he’s over 30 and I can’t force him.

He has the notion that this spider might be a recent arrival in this area (Northern California). We’re not aware of any history of this sort of problem. But people and stuff come here from all over, so just about anything could sneak in.

Any ideas?

Thank you.