Help!!

my son is 3 years old. he says amma appa,'mother','father'in tamil our mother tounge. he experts himself in operating the smartphones. he remembees the icins and all. he responds when calling his name or we say come. he smiles he laughs with us. he expresses himself .he likes to play with his preschool mates and other littleones whenever he sees them. but he cannot speak out . what might be the problem? other thing is he is the only kid in our family which conssists of me, my husband and our little one'. whats his problem . could you help plzz