Question

If a boy exhibits a sex-linked trait that is carried on the X chromosome, what must be true?

Answer 1

The boy has a X chromosome inherited from his mother.
The boy has a Y chromosome inherited from his father.
What ever genes carried on the X chromosome will be expressed whether dominant or recessive.
He will donate the genes and X chromosome from his mother to all of the daughters he fathers.

Answer 2

Everyone has two sex chromosomes, one from each parent.

Girls are XX, so when the kid gets an X (i.e., one X from Mom, one X from Dad), the kid is a girl. Boys are XY. Because they could only get the Y from Dad (he is male, so by definition his genotype is XY), you know that his X could only come from Mom.

Hope that is clearer.