You're right, it is your business. You would have to tell your boyfriend that not only do you not like him telling your business to other people but that you feel it is inappropriate for him to tell pretty much, a child, anything about you. If he is close to his nephew, he may not see anything wrong in confiding in him, but if you tell him how it makes you feel, he might stop.
so how did he tell him this? on the phone? what made him say it? how did you know he said it?