My brother is flying BOS-ORD-SJC today. He's checked-in and sitting in BOS. Weather delays seem to have grounded flights into/out of ORD by at least an hour. His original schedule was
BOS ORD UA531 Noon 1343
ORD SJC UA559 1510 1743.

BOS ORD is now at 1330 1504

I assume ORD SJC UA559 will also be delayed because of weather and he will make it. But, if he doesn't, the next ORD SJC is at 1955 reaching SJC at 2230.

Can he request a re-route ORD SFO instead?

FWIW, his fare basis is WA14NDQ