I totally sympathise with this - I'm on this part of the forum for exactly the same reason............
I am MORE than happy to be having a son, (we found out on Jan 2nd) and although all 4 grandparents are happy because baby appears happy and healthy, I knew my mum secretly wanted a grand-daughter.
I remember dialling her number in from the car park at the hospital and feeling quite mixed about telling her as ever since I announced my pregnancy, she has made it quite obvious that she'd love a grand daughter.
I know my mum inside out, and I could pick up on her disappointment when I told her, which left me feeling pretty rubbish for a day or two I have to say. I actually posted a thread in the forum at the time on this.
I had it out with her a couple of days later, and it seems to have done the trick. She has been honest that, yes, she would have preferred a grand daughter, but she is happy and that the baby being healthy is far more important than gender. We talked it all through, and since then, she has become genuinely excited about the prospect of her grandson (the knitting needles have been out in force), and just occasionally there are still comments that she makes, such as we were in Debenhams a few weeks ago, she saw a dress and said "oh well, maybe next time". Rather than let it go, I asked her about it and it did turn out she was joking.
I accept that she wanted a grand daughter - she hasn't got one, that's not my problem, but it's been pretty good actually watching her come round to the idea of her grandson - I honestly think she's over the initial disappointment.
Only you know what your relationship with your mum is like, but from my experience, if you can, it's well worth talking it all through with her, finding out why she's so desperate for a grand daughter, and even speak about your fears that she'll love the baby less if he's a boy.
Good luck with it - it's hard when parents react a certain way, especially when it's about something as important as your baby, but talking it out can help