I had a similar experience when I was a teenager and my mother took me to the doctor. The doctor said it was a normal part of puberty and that it would go away after a few months.
It went away and I was fine and I am still fine. However if you think that this could be something more serious you should take him to the doctor and check what is going on.
And did you try to touch
his nipple? Maybe if there is some kind of lump under the nipple this could be
the cause of that pain. And then again some nipples are more sensitive than the
others and some pain is sometimes reaction to this sensitivity.