A. Since both my sons said "Mommy" well after they'd mastered really essential words like "garage," "fire truck," and "apatosaurus," I can sympathize with you. I rationalized this slight with the self-congratulatory notion that I was so available for them both that they simply didn't need to learn my name. They just knew I would be there. Of course, this was a totally made-up excuse, with no basis in science, intended solely to make me feel better.
In your case, however, you've got science on your side: Your son is not expected to be speaking English as we know it at this age. Most babies are very busily babbling in the second half of the first year, and real words tend to appear anywhere between 10 and 15 months or even later. By 15 months many babies have ten words or so. If Alex is not even a year old yet, he is ahead of the curve if he has any words at all.
And just because your son isn't saying "Mommy" doesn't mean he doesn't understand the word. In general, comprehension comes before expression. Furthermore, recent research reveals that babies can understand the word "Mommy" as early as 6 months of age. A researcher at Johns Hopkins University found that babies reacted differently when they heard the word "Mommy" in conjunction with a picture of Mom than in response to pictures of other women. So even if your baby doesn't say "Mommy," he absolutely knows what it means and is getting ready to say it. (Both of your son's words are "d" words -- that sound just might be easier for him right now.)
What you can do is what all parents should do to encourage their babies' language development: Chat with him one-on-one, read him stories, and play with him, and don't worry about his verbal output. You can play games like looking in the mirror and pointing to Alex and Mommy, look at photos of yourselves in a photo album, and read stories about babies and mommies if it makes you feel better -- but if he doesn't like it, stop, because it will backfire. The key is to talk about things that are interesting to your son. If he likes dogs, talk about dogs -- talk about the types of dogs there are, talk about the way different dogs look, and get books from the library about dogs. If he is engaged, he will learn. Don't be embarrassed to use lots of drama when you speak, since that also catches toddlers' attention. And try to say "Mommy" and "Alex," rather than "I/me" and "you": for instance, "Mommy will have a cup of tea and Alex will have a cup of juice." The perspective-taking required by pronouns makes them tricky for little ones.
If you can find ways to have entertaining conversations with your son, and allow him to develop at his own pace, the word you're waiting for (and plenty more) will come before you know it.