I have no experience of attachment disorders but after reading your list had a quick search for a checklist.
Obviously I know nothing at all about your nephew or what his home life is like/has been like but so many of the things on the list I found could apply to my boys. It would take too long to go through each point (the list I'm looking at has 33 items on it) but lots of those can equally apply to autism. I'll C&P the main ones:
1)My child can't keep friends for an age-appropriate length of time. - Yes, due to social skills difficulties.
2)My child doesn't do as well in school as my child could do even with a little more effort. - Yes. Until fairly recently ds1 had fairly average marks at school. With the right help he is now several years ahead in some areas.
My child pushes me away or becomes stiff when I try to hug, unless my child wants something from me, in which case my child can be affectionate and engaging. - Ds1 only wants to be hugged strictly on his own terms.
My child has a large need to control everything. - Absolutely, as part of their autism.
My child does dangerous things such as runs away, jumps out of windows, or other potentially harmful actions. My child seems oblivious to the fact that my child may be hurt. - Yes. A lack of awareness of danger can be fairly common with autism.
My child deliberately breaks or ruins his things or other's things. - When angry, my ds has been known to break his own things.
My child doesn't seem to feel age-appropriate guilt when my child does something wrong. - Yes, again as part of their autism.
My child is impulsive. My child seems unable or unwilling to stop doing something my child wants to do. - Yes, being impulsive or having compulsions is common with autism.
My child doesn't seem to learn from mistakes, consequences, or punishments (my child continues the behavior despite the consequences). - Yes again.
My child seems not to experience pain when hurt, refusing to let anyone provide comfort. - Yes, ds1 feels little pain and has always been this way.
My child does not usually ask for things. My child demands things. - Yes again.
My child is quite bossy with other children and adults. - Yes again.
My child often does not make eye contact when adults want to make eye contract with my child. - Absolutely!
My child has extended temper tantrums. - Yes, for several hours at a time.
My child chatters non-stop, asks repeated questions about things that make no sense, mutters, or is hard to understand when talking. - Yes!
According to the checklist I should be worried about attachment disorder if I can tick five things. I've ticked twice that many without even finishing the list.
As I say though, I don't know your nephew's circumstances and they are none of my business. I'm just posting this to show you that there can be a big overlap between various types of SN.
Whatever the cause, I hope your nephew is able to get the help he needs and wish you all the best.