"I hope you have it in writing that it is supposed to slope away from
the house. "

He probably doesn't, as that level of detail is usually not specified
in a simple patio job. However, this is such an obvious and well known
concept that the homeowner would very likely win in court. He hired a
professional who he is relying on to do the job correctly. This would
be like taking your car to the mechanic for a new water pump and then
finding it leaking because the guy did not use a new gasket because you
didn't tell him to do so. Plus,.I'm sure there are building codes that
cover this too.