Male lactation???

I was at my boyfriends house and we were messing around and all of a sudden he starts squeezing my nipples really weird and he tells me he's trying to make milk come out! Obviously I was very confused, and then he told me that his do it! Then he showed me! My boyfriend made milk (or some white substance that looks like milk) come out if his nipples). Then he proceeded to tell me that all the guys he knows can do it and he was weirded out that I couldn't! I desperately need an explanation for why the heck this is possible.