This must have been an awful shock. I do sympathise with you, as a 12 year old DS-owner myself.
But I don't see any point in punishing him. Have you ever discussed porn with him and told him you'd prefer him not to go looking for it and why? If he hasn't broken a rule, how can you justify punishing him? By all means tell him you will be monitoring his phone and internet access from now on, but why oh why weren't you doing this already?
All 12 year old boys can be agressive and disrespectful. They are also becoming sexually curious and will look at "rudery" whenever they can. There is no causal relationship between the two issues. They are both the result of a massive testosterone surge which is out of anyone's control. You will not prevent him being disrespectful to you and your DD by giving him a copy of The Female Eunuch to read.
If you make an explicit connection to him between his recent agressive behaviour and his exposure to pornography you will do your son an enormous disfavour and will make him feel extremely guilty and confused.
My DH operates an unofficial PC trouble-shooting service for many of our less computer-savvy friends. I have lost count of the number of times he has found evidence that the kids (some as young as 10 ) have been accessing, or attempting to access porn on the family PC completely unbeknownst to their parents. Classic example being an unsucessful Google search string spelt
"horney lezbean biatches".
I cannot stress just how important it is to a) porn-proof your DC's computers and mobile phones, b) remind them that you are able to monitor their online activities and c) have the porn discussion with them, preferably before they are curious enough to go looking.
You can explain to him that its natural to be curious but porn isn't the best way to find out about sex because it is just acting, it is illegal for under 18s to see because he might see stuff that is disturbing and will put him off, and that acting in porn isn't always nice for the women.
And please be aware that in spite of the above precautions, our DC's will access porn elsewhere if they are really determined to.