LOL at all the people here with their heads in the sand saying the DS and his friends don't really know what it is they are looking at or saying, and that it's only hormones.
I agree with those who say the phone needs to be viewed as a privilege to be awarded by the parent and taken away, and that this situation can and must be turned around. He needs a lot of guidance and strictness and absolutely needs to know he is accountable to the person who provides the phone for the way it is used.
Calling a girl a slut is despicable even for an 11 yo. I would be very inclined to talk with the school about the atmosphere there if the children are talking in this way to and about each other. I don't think it could possible be anything but a nasty environment for girls to be spending their days in, or for boys who don't want to get involved in this sort of scummy behaviour either. The school has a responsibility to create a positive culture within its walls and if children are engaging in this sort of banter among each other then the school needs to know and needs to come up with a plan to correct it. The school should be finding out how these children are getting access to porn and they should be interested in whether it is being shown around in school.
I also think a child of 11 has no business on FB and should be reported.
I had a lot of cringemaking chats with DS in the car (so no chance of escape) on the subject of homophobia, calling people or things gay, judging people (especially girls) based on appearance, etc. It is too easy to just say boys will be boys and let them get away with this crass and offensive speech and behaviour. I feel I owe it to him to counteract the messages he gets from his environment, and also to his future partner, any children he may have, any future colleagues, make or female. I don't think a parent should stand back and let a child sink or swim in the dreck that surrounds him. You have to point out what is right and what is wrong. It is not always obvious to 11 year olds.
'Remark from other kid regarding my current pregnancy. Followed by "Bet your mum and dad watched loads of porn before your mum got pregnant. Bet you do too". "Yeah yeah LOL and my baby brother and the little shit isn't even born yet".
THIS. This is the worst, by the way. But pretty bad, no?'
I think you are right that there is something really bad there, some coldness or callousness, a lack of tenderness and warmth towards the baby who will be joining the family. If he is trying to impress the friend, then I would be afraid that he would go way too far in other areas too, just for approval.
How much spare time does your DS have? How bound to the family do you feel he is through mutual responsibility, chores he has to do, activities you share together -- in other words how many opportunities do you have or could you create for you to transmit your values to him?
I think you really need to get his dad to play a role in the lecturing and whatever other course of action you decide to take. The DS needs to hear from an adult man what is and isn't ok for a boy to be saying and doing.
I think sitting and reading it out to him was an excellent way to tackle this, and now you need to follow up with drawing him closer to the family, giving him family responsibilities, regular chores and someone to be accountable to about them, and think of positive things to do with his dad that they might both enjoy.