Our oldest son is 14 and we have always spanked him. However, the last few times he has been spanked he has developed erections. How should this be handled?
When a boy gains the ability to have an erection, he has no control over when they happen. Since erections are physically caused by a muscle clamping down on veins returning blood from the penis, things that cause muscles to tense up will trigger erections. Especially early on, a boy does not have fine control over which muscles will tense and which will not. Such fine control is not likely to show itself during a spanking and spankings do cause a lot of tension.
For this reason, I suggest that fathers handle the spanking of teenage boys if such spankings are needed. The erections themselves should simply be ignored. They are a normal physical response to being spanked.
However, since you know that he has reached the age of sexual capability, it is time that Dad sits down with the boy and begins teaching him about reproduction and his new responsibilities to keep himself pure. See Growing Up in the Lord: A Study for Teenage Boys for a study guide.
See also: Does my son enjoy being spanked?