S.S.
My son will be 3 next week and it wasn't until VERY recently (last 6 months or so) that he has called us Mom and Dad with any regularity. And since then he's gone through different versions. I was "Mom" for a while and now he's trying on "Mama" for size. As long as he's understanding the difference, I wouldn't worry if he's using the words. He will in his own time.