speech problem

my son is 20months old now. he is not talking well. he is saying Amma appa Thatha paati Apple ball car auto no no Tha like tat. but he s not saying any other words as well. he s understanding everything that what we r saying r asking to him. but he s getting trouble t talk.. when I asked him to say this means he will say ah mm like this... my cousin's son is same age, he s talking nd answer so gud but my son is not talking lik him.. so I'm worried.. when he will talk fluently ..