What Causes Painful Nipple?

My boyfriend who is 38 is complaining that his left nipple is really hurting him. When he squeezed it, thinking that it could be a pimple, he threw up from the pain. He works outside in very cold conditions. I am wondering if the problem is due to the cold or is it something serious. He can't touch the area or sleep on the area because it hurts him. Is there something we should try to help allievate the problem.