Dear Rabbitman, I am probably younger than you, but I can relate to your girlfriend. My daughter is 5 years old and has autism. She is non-verbal and has many sensory issues. Much of what you said bothers me, and here's why. There is so much more you could be doing for this young man. Since you have a diagnosis you can get a PCA for at home to help you. A PCA is a personal care attendant paid for you by your insurance company. We have one for my daughter and it has made a world of a difference. Now when my daughter has a meltdown or needs to use the restroom while I'm cooking dinner I don't have to stop what I'm doing to deal with it!
My daughter also has a service dog. He is a beautiful, well trained, Labradoodle. Guess what, it is against the law for your landlords to deny access to a licensed service animal. It doesn't matter whether they understand the difference. If they try, call the ADA and press charges against them. 800-514-0301. Your girlfriend's son would not be required to be the dog's handler or caregiver. You or your girlfriend can do that. You never know, she might be willing to do it if it would help her son. Also, the PCA can help too. Here are the official laws regarding service animals. http://www.ada.gov/qasrvc.htm
Teddybears4ever2 is right. His mind is underdeveloped. He doesn't think like a regular teenager, my guess is he thinks more along the lines of a young child. It is hard to treat someone who acts like a young child like a young adult. He is going to grow up much slower than his peers. I'd rather get some sleep then stay up all night trying to get my child to sleep in their bed. It takes a toll on you. Good luck.
He has a paraprofessional working with him in school and he is a very good student, but given his autism, he is reclusive and antisocial in his behavior. He doesn't like to be touched and unless he approaches you first, don't even think about approaching him because he'll put up a fight, start pounding his head with his fists (as seen in "Rain Man" with Dustin Hoffman's role) and scream uncontrollably. Now the paraprofessional cannot be with him 24/7, so it's up to his mother and I to be his "paraprofessionals" at home. I know he is a good kid, wouldn't harm a fly, but I also want to ensure that he sleeps well with as minimal discomfort as possible. Regarding a service animal, that might not work because I am not sure if the building she lives in allows dogs, and it's always a problem convincing these ignorant landlords about the differences between a pet and a service animal. Also, the dog needs to be walked and cared for, and I doubt he can live up to the responsibilities nor does his mother have the time to care for a dog.
Imagine that your body is that of an adult, but your brain is one of a child. That is what this kid is going through. His mind if that of a child, he sees things like a child would, and acts like a child would. Maybe you can offer to help get him a service dog or cat, and he can sleep with it. That might help, otherwise there is always the option of getting him a stuffed animal to keep him safe.
Sure she wants to break this habit...she knows in the long run it's no good for him to use her as a crutch for his fears like this. I spoke to her about this and she will try to gradually ween him off of this behavior because as he ages and matures, it's only going to get worst. And to the presumptuous fool who called me "selfish and inconsiderate", do you think I am that as I care very much for the kid and want to see him break free of this habit and his fears because it pains me to see him like this? I would not be addressing it if I didn't care. Trust me, I understand what autism is very well and don't need to be lectured to on it, given I am older than you and one of my nephew's has been diagnosed with mild Aspergers Syndrome, so don't assume I am some clueless kid who knows nothing.
"I have criticized her..." is not going to be a helpful approach, ever, in any situation. Criticism leads people to dig their heels in more firmly in their position and effectively negates any hope of communication. Tough love doesn't cure autism or nightmares. You might disagree with how she is handling it, but you come across as selfish and inconsiderate. I would suggest learning more about autism, trying to be a little more understanding, putting an end to the criticism, and seeing if that yields more positive results and a girlfriend who becomes more interested in your feelings once she feels her own are respected.
In your post you describe why the boy sleeps with his mother so there are reasons.
Does your girlfriend want to break this habit or is she comfortable with the current arrangements.
This needs to be handled very sensitively, the boy is likely to be very upset and find change hard to deal with. Getting him out of his mothers bed would likely be a long and difficult process maybe with many set backs. Do you have the patience and understanding to support your girlfriend with this? An occupational therapist can help provide a plan and support your girlfriend with making these changes.