Is it a sign that my son is trying to speak?

My son is 20 years old. He's severely autistic and nonverbal. He needs me to dress him up and give him a bath, but he can independently sit on the toilet, walk or run in the garden, play with our dog and so on. In these 20 years I've understood his needs by the expressions on his face and also by the body... show more My son is 20 years old. He's severely autistic and nonverbal. He needs me to dress him up and give him a bath, but he can independently sit on the toilet, walk or run in the garden, play with our dog and so on. In these 20 years I've understood his needs by the expressions on his face and also by the body language he has learnt at school so far. He usually tries to imitate the birds' singing or he just simply screams when he's sad, angry or having a melt down, but recently he keeps saying a world like "mama" when he wants to go in the garden. I've talked to his doctor about this and he says that there's not much possiblity for a nonverbal autistic person to learn how to speak after the age of 15/16. But I just can't believe him or I don't want to believe him. At least I think my son is trying to learn how to speak.
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