Is it true that a mans sperm cells determine if a child will be a male or a female?

Yes, it is true that a man's sperm cells determine the sex of a child. Sperm carries either an X or a Y chromosome; if the sperm fertilizes an egg with an X chromosome, the child will be female (XX), and if it fertilizes an egg with a Y chromosome, the child will be male (XY). Therefore, it is the combination of the sperm's chromosome and the egg's chromosome that ultimately determines the child's sex.