As someone who has worked in both changing legislation/education and directly with young people around the issues of consent and internet use, I’d say (with the greatest respect) that a lot of these messages telling you to directly condemn your son are (understandably: it’s emotive) a bit reactionary and ill thought through.
You want your child to have complete trust in you and to come with you around all these kinds of issues, if you just tell him off without contextualising it you’re effectively closing the door. Despite being 15, he is still a child and there may be lots of things at play here: friendships, power struggles, lack of understanding about consent and so on.
I would calmly sit down with your child & your partner (if there is one) and speak directly about consent, friendships, peer pressure, respect, the law, safe Internet use and so on. If you just get cross, take his phone etc that isn’t going to help him, or help keeping an open dialogue for the future.
This is an opportunity to have an open and frank conversation about all these things- not merely condemn him. It needs to be made clear that this is considered child pornography and it’s against the law, and to make him truly understand how this made the gf feel. Thinking about the repercussions of his actions (and inactions) and how him being a passive bystander / not being an ally have had such terrible consequences for her.
It’s hard to ascertain what went on without seeing the actual “chat” so I would take your son’s phone and contact the other male child’s parent and get them to do the same. Then look through the messages so you can get a better picture of what happened. Would the other boy (the bf) parents be open to discussing it with you do you think? Are they approachable? As it sounds like they really need to up their game in terms of educating their child around safe, consensual and appropriate relationships, esp if he is in a relationship. I’ve posted some resources below which you could share with them.
Your post suggests you have spoken to the gf parents, and that the gf directly told them that it was the bf and not ds. Irrespective, your son should have been an ally and you should speak to him about stepping up to protect others who are in a vulnerable position. This is across all walks of life, not just this particular instance.
I would also get your son to make a face to face apology (with her parents and you as chaperones) where he should outline that he knows what he did was illegal, wrong, threatening and frightening for her and stating that he will stand up for her and shut it down if it ever happens again.
It’s not really useful to just close down and shame a child without putting in place the emotional and educational infrastructure to make them understand what they did, why it was wrong and how to manage this situation going forwards. I’m not taking away from the fact that the gf is the victim here and she should be apologised to and both male children severely punished, but there are ways to go about this.
You might find it useful to look at resources from the below. I would also share these with the parent of the gf as it has help on how to speak to your child if they have been on the receiving end.
NSPCC - www.nspcc.org.uk/keeping-children-safe/sex-relationships/sexual-behaviour-children/
Internet Matters - www.nspcc.org.uk/keeping-children-safe/sex-relationships/sexual-behaviour-children/
**this link (below) has a direct section on sexting and on what to do if your child is the perpetrator :
www.internetmatters.org/issues/sexting/dealing-with-sexting/