A little supplemental info to the good stuff that’s already been posted . . . .
While we’re a long way from a definitive understanding of autistic spectrum disorders, the prevailing current model is that autism entails a difficulty or inability in forming a “theory of mind” regarding other people – that is, understanding that other people have knowledge, beliefs, desires, intentions, etc. that may or may not be the same as your own, or even that they are people and not merely objects. This makes it very difficult, if not impossible, to understand or predict the behavior of others. Most of us begin doing this at a very young age and do it unconsciously with increasing sophistication as we mature. These abilities develop more slowly or not at all in people with autistic spectrum disorders.
The analogy of being an alien is a reasonably apt one – everything that happens around you would be more or less incomprehensible, and fear, anger and frustration are common human responses to such situations. Things that are familiar and predictable become essential lifelines to coping with the world; that which is novel or unpredictable is threatening and confusing. This difficulty in formulating hypotheses about what other people want or are about to do seems to be tied in with a difficulty in filtering and selecting the relevant and important bits from the vast range of sensory input with which we’re constantly bombarded; thus noise and tactile sensation can be particularly distressing, as they compete with everything else for conscious attention. Indeed, some theories of autism spectrum disorders regard this as the fundamental attribute, classifying them as “sensory integration disorders”; i.e., difficulty in processing sensory input “appropriately”.
Thus, the hallmarks of autism: impaired to non-existent social and communication skills, preference for routine and familiar objects and (where communication isn’t too severely impaired) topics of discourse, difficulting in understanding and/or using humor (which depends on establishing and then subverting expectations), preference for inanimate objects over people and social interaction, stereotyped and ritualistic behaviors, and increased sensitivity to sensory input (noise, tactile sensation, light, taste, etc.). Some people classified as autistic are extremely verbal, in that they possess a normal or even large working vocabulary and can and do talk at length (even extreme length) about certain topics, but their verbal interactions with others tend to be one-sided monologues, lacking the back and forth flow that characterizes “conversation”. They may say or do things that would ordinarily be considered inappropriate in a given situation, because they don’t understand or appreciate the effect their words or actions have on others, or even that they do have such an effect.
Of course, individuals possess these traits in widely varying degrees, and there’s no real consensus on where the dividing lines between “typical” and “autistic” are; some even argue that the difference between “autistic” and “male” is one of degree, not of kind. My seven-year-old son, for example, will talk your ear off about whatever his pet topic at the moment is, and seems to have trouble appreciating that his interlocutor may not share his degree of interest in the topic. He often doesn’t recognize when someone is trying to change the subject or move on to doing something different, which gets in the way at times in his interactions with others. He’s also more sensitive to tactile sensation than most people. He’s often reluctant and apprehensive about new experiences. Yet he also has a quick wit and an appreciation of humor that indicates an ability to understand other people’s expectations that goes far beyond what would ordinarily be expected in someone with autism, and he’s a genuinely kind and empathetic kid a lot of the time. He’s shown a tendency to ritualized/stereotypical behaviors in the past, but he seems have developed an ability to recognize and moderate them. His interests tend to be intensely focused in the way that’s characteristic of autism, but he also exhibits an eagerness and willingness to please others that isn’t. Having observed and considered his behavior as objectively as I can, as his parent, I don’t think most diagnosticians would conclude that he meets enough of the DSM-IV criteria for autism to be classified as such, he certainly comes close on several of them. He would more likely meet the criteria for Asperger’s Syndrome, an autism spectrum disorder that typically does not involve significant language impairment, but even then he’s borderline.
The advice from others to take your cues from the mother is good; she’s going to have a better sense of what works and what doesn’t with her son. Hopefully the other information here will help you understand him and his behavior.