What Should I do as far as getting in touch with my ex-girlfriend about our son, when I don't know her address?

Ok. I have a 3 yr old son up in Springfield, MA, & I have not heard ANYTHING from his mother/my ex-girlfriend in MONTHS and it is really startin' to irritate me, & drive me up a wall. Not to mention, I am growing very much weary of the fact that I don't hear from her in regards to our son. She fails to keep in touch with me, & when I make ANY attempts to keep in touch with her, she never answers her phone. The only way I can get her to answer her phone is if I call her blocked, and she has a fit each time. I do not know her current address, yet I do remember her former address. The last time that her & I spoke, which was around November of last year, she had informed me, that she plans on moving out of Machusetts in June of this year, yet wouldn't specify where. It is really startin' to upset me, & worry me, & I truly DO NOT know what to do in order to get in touch with her in regards to our son.. Hell, I haven't even seen him since March of '07, when her & I parted ways on what was supposed to be a TEMPORARY split in her words, which ended up being a permanent split due to her own selfish desires. Anyways, I am just worried about my son, & I don't know how to go about getting in touch with her without ruffling her feathers and causing even more friction between her & I, for everytime I threaten to take legal & evasive action against her, she threatens to do the same. And what makes this matter worse, is even though her & I BOTH know that the child is mine, he does not have my last name, due to her bein' married still when she gave birth to him back in '06. So again, what do I do?