Have a problem with my nephew's mother.?

I have a 4 year old nephew that is cute as a bug but doesn't act like a 4 year old. He is very stand offish and doesn't speak loudly or some times at all. His mother doesn't take the time to mother him like she should and she expect everyone else to raise him. This is my husband's brother's son and he isn't in the picture at all. My husband and I have three boys and just had a baby girl 5 months ago. We have other nephew's that are three years old and they speak clearer and understand things way better than this child. I have taken him to be check for hearing and other test and they came back normal. The school said that they want to test his comprehension and that was this summer. The problem is that the mother won't take him to the appointments plus she is out doing things rather than taking care of her son. It gets to be very challenging when every weekend my husband picks up this child and he will speak a little bit to him and not at all to me and my boys. The boys are 14,11, & 7.