Anne,
I too am a stepmom, and have been for several years. My son has never called me mom, but my daughter does sometimes. They are 19 and 16 respectively, and I've been in they're lives since he was almost 11 and she had just turned 8. The first thing I'll tell you is to ignore anyone refering to you as anything but a "real" mom. A real parent is someone who walks the walk - not just talks the talk. It's a lot of work being a stepmom - and it can be very rewarding, and very painful (sometimes at the same time).
As for your stepson calling you mom, I'd just say "that makes me feel good when you call me that" the next time he does it. Then, leave it at that so he doesn't feel like you expect him to do it all the time. He's young, and there must be a reason you and your husband are fighting for custody, so as long as you are a mom to him in the most important ways (I'm not discounting his biological mom here, just refering to your duties), it doesn't really matter what he calls you as long as it's with respect.
Both of my children have told me that I'm more of a mom to them than their 'real' mom is. And it only makes me proud on the end that I know they recognize me as a mom to them. I feel bad that their biological mom has never stepped up to the plate fully other than to use the kids for what she can get because of them, but I know that my husband and I have given them the best that we could (as far as morals and such, I'm not talking material goods), and we given them what they needed when they needed it (mentally and physically).
I smile when I'm called mom, but I know that Donna carries just as much with it.