The probability that Mr. Smith's other child is also a boy is 1/2 or 50%.

This is because the gender of each child is independent of the other child's gender. Regardless of the gender of the eldest child, the probability of the other child being a boy or a girl is the same. So, if the eldest child is a boy, the other child could be a boy or a girl with equal probability. Therefore, the probability of the other child being a boy is 1/2.