I cannot condone any man making a decision to abandon his child, however the conception came about, as the child misses out through no fault of their own. Your BIL is behaving badly and I can understand your disappointment in the behaviour of him snd your PIL.
You do sound a little over-dramatic/involved though. Yes, it isn’t palatable behaviour but being “devastated” seems a bit OTT if I’m honest. If I were you, I would let the dust settle before putting your oar in. If anybody should say anything it is probably your DH- though even then I think interfering is unlikely to help the situation at present. Say your piece if you must, but then leave it there.
You could contact the ex-partner to see if she wanted to facilitate contact with your DH as uncle, but I’d also go carefully there. It’s easy for others to say just cut off contact with your PIL as you disagree with their stance on this- and it may be that this is what you feel you must do/want to do- but I’d also be careful of flouncing off in disgust and in high dudgeon because that does have repercussions for your DH. Additionally, it may be that if the situation does change/attitudes change and there is an thawing of relations between BIL or your PIL and the ex-partner/mother of the baby, but there is an unhealable rift between pil/bil that (which may or may not bother you), which leads to you pushed out of the child’s life/family. I have seen something similar happen (though not about a baby) and if one party had just made their point and let it lie, rather than get in their high horse (they did have a point, but were not totally in the right), things could have been repaired. Unfortunately, this has led to becoming distanced to some degree from wider family as get togethers can be awkward etc.
I genuinely think you can make it known that you disagree with what is going on and leave it there- perhaps also putting some distance rather than cutting all contact- without turning it into something from which there is no going back.
As an aside- I also disagree with some posters re whether a man can be “tricked” into fathering a child. I think it can happen- yes, no contraceptive is ever 100% effective so if you really don’t want to a baby, use more than one. It’s also true that contraception is not only up to the woman and that “I was tricked” is an overused “defence” (which in no way resolves the father of his responsibilities). BUT if you are in a relationship with someone and they tell you they are using contraception that is highly reliable when they aren’t, then I would say that I’d being deceived. Obviously, if it’s a ONS, then less of a defence.
I think many of us will know women who have deliberately got pregnant against their partners wishes- I know 2 women who definitely have. One wanted another baby but her husband wanted to stop at 2- so she stopped taking the pill without telling him. Another misguidedly got pregnant when she felt her relationship was failing, hoping it would bring them closer together- she stopped getting depo injections. Worked out on fir former, not well at all for the latter.
I have no idea whether OP’s BIL was tricked- though his actions are irresponsible and unfair to the child- but if he was, I can understand why he might behave unreasonably out of anger and frustration. Even if I don’t condone it.