I would ring social services.
At the beginning of this thread my thinking was that perhaps this child has a specific sn which would explain the obvious speach delay etc.
But now I'm wondering whether his delays are actually because he has not had interaction at home for the past three years.
He's in nursery now so obviously is getting interaction from others, but what was the situation before he went to nursery?
Children learn to do things by example. So for instance if no-one ever talks to a child then the child will never learn to talk. When I was at school I knew a child whose parents had carried him everywhere until he was 5, because they were so afraid that he would run into things (he was VI) that they didn't let him walk. Consequently the muscles in his legs were so underdeveloped he had to undergo years of physio/OT to help him learn to walk properly. Equally someone else I know whose parents are both profoundly deaf. As a result he grew up with sign language, and so didn't learn to speak until he was 4 and went to nursery.
Maybe this woman needs some help with interacting with her child. Or maybe she's just neglecting him and ss need to intervene.
There was a woman at one of the toddler groups I used to attend who, because of severe learning difficulties, had real problems interacting with her dd. She didn't talk to her/never did anything with her and as result the little girl's development was severely impaired. Ss and the nspcc worked very closely with her for about 2 years iirc to try and help her wrt her parenting. Unfortunately things didn't work out and her dd was taken into care. Sadly this was seen as the best course of action for all concerned, and the little girl went from strength to strength after that. She is now being brought up by a member of this woman's close family, so she still has a relationship with the child, she is unfortunately just not the best person to be bringing her up iyswim.