It seems as if the short one
is probably the grandson of the tall one because a grandfather can most likely
still be considered a father (hence the name—grandfather). However, like the riddle mentions, the short
one won’t be considered the son, per se.
As for when the phrase “you
is” would ever be correct, the answer is never.
In order for there not to be a subject-verb agreement error, the pronoun
“you” always needs a plural verb (such as “are”). And because “is” is a singular verb, to say “you
is” is a subject-verb agreement error.
