As down syndrome problem is the cause of the mental disorder. A mental disorder child is also not capable to communicate well so it is more difficult for him to survive in the talkative world. A speech therapist can help him by developing the communication ability in your child.

Yeah, your child is late in speaking well, but he can learn now how to develop communication skill. 2 year old child is able to speak 50-70 words and able to joint these words and can make sentences. You are not too much late, you can help your child in developing speech by taking help of a speech therapist.

15 month old is enough big to talk some words like Mama, Dada. A 15 months age child can speak 10-15 easy words. If your child is not able in talk accurate words then there is some problem in your kids communication skill and you should take help of a best speech therapist.

I am totally shocked with this that fathers influence child language development more than mothers because a child live whole day with mother not with father. mother is the first teacher of a child.

21 months child is enough capable to speak 30-40 words easily and can joint the 2 words and can a small sentence as well. If your friend child is not speaking single words then your friend should have to give more consideration on her speech. They should take a help from any best speech therapist.

As the mothers are the first teacher of a child. Child starts to learn everything from his parents. so it is very nice that you teach him about how to communicate well. Talk with them your daily schedule in a easy language so that they understand what you want to say.

In preschool he may start communicate well because a child learn to speak when he saw other children of his same group. As the mothers are the first teacher of  a child you should do some extra efforts to help him in communicate.
You can read a story book for him and use the picture books and sing nursery rhymes for him.
Talk him and tell him your daily schedule in your easy language so that he will try to understand that.
Choose a best speech therapist who is professional and have experienced.