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A thought occured to me overnight.

While at playgroup with my children, a mother told me that her youngest had started to deny doing naughty things. She would be naughty and when her mum told her off, she would say "it wasn't me".

Maybe children learn to tell lies as part of early development, which becomes eventually, the social skill of using the "white lie", as well as lying to avoid trouble.

Since autism involves late development of social skills and is also ascociated with an inability to use the "white lie", maybe, when an autistic teenager has a period of lying about everything, they are learning to lie.

I certainly have become, say, more selective with the truth over the years. I still favour the truth because hints and body language are lost on me.

 Perhaps it is just a late development of the social skill of lying, and needs to be practised, as does any other skill. If he gets angry about it, when you challenge him, he is perhaps, more angry with himself for failing to get it right, than with you. 

Hi Dr3am3r, I have to admit to lying frequently in my teens. I wanted my own way. Rules to me were guidelines to be broken if I had a reason. Eg if I wanted to go to the library in school hours, I went and made up an excuse, such as returning books for a sick relative, if caught. Thus, to me, it was not wrong. You could try just accepting what he says, and let him suffer the consequences, eg no pack up, if he lies about making it, or no clean clothes if he misses the wash. We can do too much for our children.

Is your son punishing his brother for percieved misconduct? If so, he may see it as good rather than bad. Or telling you what has been done to him, maybe, which may justify it in his eyes?  I

 think it can be  a way of telling you what was happening, without telling tales, which is taboo among children, and he might have been hurt again for reporting an earlier issue.

The lying question has come up on other threads recently. If you are constantly appearing in the wrong, the demoralising effect on self esteem means that anything, even a lie, avoids the damage it does.

But it is more complex. You say on one occasion he was stabbing his brother with a pen, till he drew blood, but denying he was doing anything wrong. Are other people stabbing him with a pen, or doing other bad things to him, for which he has no means of defence? - at school? in the neighbourhood between school and home? etc. Also if he has difficulty gauging other people's responses, he may be searching for a more meaningful reaction from his brother (and indeed some people on the autistic spectrum experiment with others to better understand what is happening to themselves.

He may be experiencing a lot of pain that you aren't aware of, such as through higher sensitivity or greater difficulty processing it. He may therefore think pain is normal, and not see inflicting pain on others in quite the same way as you might.

Honesty and "telling tales" seems to be a common element to autistic spectrum. It is one of the things that gets you into trouble with your peers (confession here, I told tales a lot - that is was, I guess still am, pedantic about the rules).

It is easy for non-autistic people to break the rules, because you can play off other people, by looks, facial expression, when you've gone to far, so it becomes a game. If you haven't got that social skill, you daren't break the rules. And why should others be free to break them if you aren't?  Also Theory of Mind issues, understanding the social background to rules, is a limitation.