I think you have handled this in a perfectly good way tbh.
yes it was a shock when you first discovered it, but now you have had a chat with your ds, brought it all out into the open and let him know how you feel about his comments. I don't really think there is much more you can do.
FWIW, I do think a lot of this sort of thing is all about fitting in with their peers, and most well bought up children might write these things, but not actually mean them. The fact that your ds is a lovely boy at home who is excited at the prospect of having a baby brother, has teddies on his bed and loves his mum
is the thing to hold on to, along with all the nice conversations on his phone, NOT the odd text that make your skin crawl!
I recently discovered text messages on my ds's ipod (aged 10, yr 6) saying - 'you are a cock sucking retard and I hate your fucking guts'
. In all, he had 25 texts all of a similar vein. Luckily, ds realised the (same aged) boy who sent these was an idiot, who deserved no respect whatsoever, we showed the school who dealt with it appropriately. BUT, the shock i felt when reading this - well, my God!
It was, however, a good thing that it happened because it meant we could have frank, honest conversations about text/phones/inappropriate language/respect etc etc and ds is now much more open about 'stuff' - iyswim. Hopefully, all this coming out will be a good thing for your ds too.
PS - do 10/11 year old pre-teens really wank?? I sometimes catch a glimpse of ds's willy and it is such a maggot, his hand would really be too big and just slide off the end as it moved up and down. Surely it needs to be, well, just bigger!! 