J.P.
I have a close friend in a similar situation. She is a wonderful person and a fun, caring mother. She often feels bad that her child is much more attatched to his grandparents. I never have the heart to point it out, but she has created the situation herself by allowing her parents to take over from the beginning. It was their pleasure to fawn over a baby, and very easy on her to enjoy the freedom of a live-in nanny situation.The problem when you share parental responsabilities is that you share parental attatchment as well. I agree with some of the other posts - if you want to be the one your son comes to and is attatched to, then you need to work hard to meet all of his needs yourself. If he wants a snack you get it, and make it fun. Same goes for bath, potty, playtime, everything. Sure, many kids just have to wait if 'mommys busy right now'. But, yours doesn't have too because grandmas there. You can't ask her to ignore him if your busy, how can she? So, you'll have to deal with the situation until you are one day on your own or work really hard to be available when he needs you every time.
I know this is a difficult situation. It's hard to be the disciplinarian and in charge of the tough stuff but not get to feel the reward of being number one. Good luck.