If your dad is cool and he says it, he's pulling you aside to have a
confidential "coaching" moment, while saving you the embarrasment of having
your friends hear the whole subject.
Can also be said to someone about your own age who is being a bit naive, for example about a girl who might be bad news, but who your homey is starting to get caught up in. You might say to him "can i speak to you son" as a way of letting him subtly know he might want to talk to you because there might be something
you know that he might want or need to know.