How can I make my 15 month old son call me mommy?

He probably has a code word for you that you just aren't able to understand. He loves you, of course. My daughter said "daddy" at one ( oh and I was a bit perturbed, believe me, lol, though I was happy to hear her talk) then "sadie" who is grandma's dog, "pie" for papaw, then some other words like ball, no, who's that, what's that...then finally before she turned two, she said "mommy, mommy, mommy!" and ran to me, and it was like the best feeling in the world! He'll say it soon enough and believe me, it'll be worth the wait!