I think he stirred up sediment from his water heater somehow and it has clogged up a point that is common to all the outlets.
I'd put the WH into bypass mode, open the low point drains, remove the check valve at the city water inlet then get a "water thief" and go to the kitchen faucet and put water into the system. Give it a backflush.
Wright and Penny
(with Fitz and Lizzie, the camping kitties)
2010 Tundra 4X4 5.7L V8
2014 Rockwood 2604WS
Life is a cruel teacher. She gives the test first; the lesson then follows.