Is child of bipolar mother special needs?

My aunt has been bipolar since she was in her late teens. She has three children, all boys. The two older boys, twins, are fine developmentally. Her younger son, who is 8, seems to be developmentally disabled. He has been held back a few years, doesn't interact well with others, and has trouble expressing himself verbally. He throws tantrums often when he is misunderstood or for no obvious reason. When I compare him to my four year old neighbor, he seems to be at her level. The only thing is, that his mother seems to be in denial about it. She blames the schools that he goes to, and gets very upset when someone hints otherwise. I need help. We are a fairly close knit family and I don't feel that I can watch him not get help, but on the other hand, I don't think its my place to butt in. Any advice?

[Sally44]

He might be on the autistic spectrum.  This can range from severe to mild.  If he has problems with speech and social interaction that is two areas under the diagnostic criteria required for a diagnosis of an autistic spectrum disorder.  If he needs routines or shows other rigid behaviour and gets upset at change, unexpected outcomes, transitions then that is another criteria.  Does he appear to have any sensory issues?
I think the first step would be for his mother to arrange for him to be assessed by a speech and language therapist who has experience of speech disorders as well as speech disorders associated with developmental disorders (such as autism).
It is the Speech Therapist who is responsible for both speech and social interaction and play skills as these always go hand in hand.  If you cannot communicate well, or understand what is being said to you, your play and social interaction/communication skills are bound to be affected.
A good Speech Therapist should be able to indicate whether a child is showing difficulties typical of the autistic spectrum, and she should then refer the child on for further assessments by other professionals.
The reason a diagnosis is important, is because a child on the spectrum would need to be in a school that has experience and expertise with children on the autistic spectrum because they tend to learn differently and they need to be taught social and play skills.  This maybe a mainstream school that has a higher staffing to pupil ratio if he is high functioning, or it maybe a special school environment, but an Educational Psychologist would be able to advise on the best type of school.