your drain is plugged full of deposits.
Have him try again with the cold water valve OPEN, sometimes this will aid in BLOWING the debris out of the valve and tank.
Sometimes even this wont work.
On occassion…(depending upon your drain valve..) you can feed a thin wire backwards through the drain valve to break the blockage.
If all else fails, you will have to siphon the water out with some rubber tubing from the hot nipple.