Welcome JuneBug!
You are in a difficult situation, with your family under pressure from the start. It is not unusual for former Witnesses to return to the organization when going through life changes or a difficult period, such as a marriage, childbirth, or a serious illness in the family--you are dealing with all of these
This relationship will be difficult for you, a woman, because the Watchtower organization teaches that the wife must be submissive to her husband. Your fiance will be seen as "spiritually weak" if he does not have the children and you with him for every meeting at the Kingom Hall, regularly going out for "service" (the door-to-door preaching), and attending several weekend assemblies and conventions each year.
Also, think about your son's relationship with your side of the family. The message at the Kingdom Hall is that all non-JWs will be anihilated in horrible ways at Armageddon, which will happen Really Soon Now. That includes Grandma, Grandpa, aunts, uncles, cousins and you (unless, of course, you become a Witness). Until doomsday arrives, you and your family will still be called "worldly" and "bad associations".
That is fine with me, until he decides that we're not having sex until we get married (supposedly June 30, 2012)
Yes, he has to cease pre-martial sex (and any other sins they know about) to get reinstated as a Witness, which is a slow and somewhat humiliating process. The time-line suggests he wants to be reinstated before you marry. Have you discussed where you will be married and who will officiate? He may be planning on the Kingom Hall and an Elder for that, which means he must be back in good standing, probably with you following him as an "unbaptized publisher". Going to the meetings with him is the first part of that process: once you are attending with any kind of regularity they assume you will join them in due time. The pressure to come into the organization may be so subtle that it seems like nothing, but it will be very persistent and difficult to resist in the long term (speaking from personal experience).
Please visit the Freeminds web site, and read the articles under sociology-marriage and sociology-women. Then consider carefully if this is the life you want for your son and yourself. Mixed JW-non/JW marriages can work, my ex-wife and I made it for over a decade. You can't make this work by yourself, he must show as much respect for you and your beliefs as you do for his. It takes extra effort and accommodation both ways to make that happen. Be sure you both are prepared to live up to that committment; don't say "I Do" unless you both really mean those words.
I wish you the best, whichever decision you make.
