Dog will only go on grass, never concrete so, what should I do to
encourage
the grass to grow .... ?

MAKE the dog go on the concrete by fencing off the grass and praising the
dog with rewards when he does go on the concrete. There is no such thing as
a badly trained dog, only a badly trained owner.

The plot of grass about 6 feet by 8 feet was done especially for the dog,
but I would like the grass to grow evenly.

If it is there for the dog, the grass won't grow.
The choice is yours.

End of story.