"Da Da", "Pa Pa", and very rarely "Ma Ma"

It must be something about the tongue or the laziness of the brain. Ethan says Da Da about almost everything, and recently, he said very accurately "Pa Pa" to JK when he sees JK go pick him up from work. Me? He rarely utters a word but just glances at me, happily with a smile. At times, he would jump with his arms up in the air in wild cheers saying, "Da Da Da Da Da Da". Rarely, or may be I should say, not yet, had he said "Ma Ma" when I go pick him up at grandma's. Interestingly, as I recently noticed, when he is in the doctor's office and Daddy is stripping him down to his diaper, he desperately searches for me with open arms and teary eyes loudly saying, and crying, "Ma Ma, Ma Ma, Ma Ma". So he knows who I am and what he should call me. But it appears that only at desperate times is his brain triggered to utter those wonderful sounds that no doubt put a smile on my face. When will this willful kid call me "Ma Ma", which I rightfully deserved to be addressed?

JW