I read somewhere that it means they subconsciously wish they were touching your breast. But who knows. lol I would take it as a positive sign of interest though.
It could mean something or nothing...I know, helpful aren't I :-)...I'm a pretty touchy person but by the time I am that way I have some sort of affection for them but it doesn't necessarily means it's romantic...Though I've learned that many people can take it that way; and have had to learn how to read that in people. Watch what he does with other people...What are the other signs, eye contact, pressure of touch, body position, does he squeeze or touch? I think that by watching him you can learn a great deal. Best of luck. Cheers!