18 months old not speaking much. Him.

Hello sir/madam..my son has just turned 18 months old..he is very active, curious, playful, social with the people he knows..only cries if any unknown person picks him..he understands and follows mainly every command that we give..acts intelligent in many situations..he says mama, dada, toto,tata,nana , only these few words..i feel mama is the only meaningful word that he uses to call the mother..but he says mama to everyone in the fanily if he wants to get something done or want's something..he doesn't say papa to his father or dada dadi to his grandparents..he said papa a few times only..and now when i ask him to say papa, he always says mama instead and smile..he has a screentime of 30-40 mins in intervals daily as we live in a joint family, so his grandparents play rhymes for him..he has achieved all other milestones..but this is my biggest concern that he isn't speaking till now..what steps can i take for him..pls help as i am extremely worried