It must be so dreadfully hurtful to hear him speak like that ..... as if you and your relationship isn't worth making an effort for.
Of course he has choices. There isn't a gun at his head .... well, maybe not a literal gun, but metaphorically speaking he's convinced himself that if he "upsets" his kids (and/or his ex) he risks losing contact with them. My DP is similar so I do know how you feel .... though he doesn't claim to have "no choice", rather he pretends not to notice bad behaviour (didn't see it, can't discipline it), or downplays whatever it is, and/or makes ridiculous intelligence-insulting excuses like the precious ones are stressed/ill/tired etc. Anything to avoid even the mildest form of confrontation such as a reasonable chat pointing out the error of their ways, let alone heaven forbid a full on telling off or punishment. Personally I feel that if his (teenage) kids are so fickle and shallow that a mild reprimand would have them stomp off in a huff then that says a great deal more about them than it does him and TBH who'd want the company of such spoilt brats ? (... at this point a thunderbolt descends on my head). However .... these men seem to be utterly rendered paralysed by the thought of losing contact (in reality, it'd probably be temporary if it happened at all, because right now, his kids are at an age where DP and his family a quite a convenient source of "stuff", money, presents, activities and so on) that I believe they just don't want to face up to the truth of it all .... both what their kids are really like (rude, spoilt, manipulative, thoughtless) and indeed, their part responsibility for how they've ended up (because that would be an embarrassment and guilt inducing). So .... it's far far easier to concoct this fantasy world where everything's apparently "fine" and where anyone who dares to disagree is shot down in flames.
Which would all be very well if these men were single .... rod for their own back and all that. But personally, I find it appallingly selfish and arrogant of them to inflict all the repercussions of this style of "parenting" (ha ha) on other people - most usually of course their partners and any other children in the household.
Your DP is totally wrong by calling you weak. Talk about projection! A weak, meek person would indeed put up with it all. Those of us who protest - which isn't often easy because you just know that even with the most careful tact, the most considered phrases and careful choice of words, a horrid row will usually ensue - are actually strong, because we don't like watching injustice in our own homes and speak up about it. In all honesty, if it were possible for me to do so I'd be off and in a strange way, and I hope I don't offend you, I actually envy your position because, if I remember correctly, you have another property which you could eventually leave to go to. In your shoes Redhen - and I think your situation is far worse than mine, not least because of numbers - I'd cut my losses and start making concrete plans to go. You've given this man so many opportunities to put things right and it sounds like he has absolutely no intention of ever doing so. The declared non-discipline approach is just crazy and will only make things even worse. It may be "just" one of them now, but I can't see that the remaining kids would accept their sibling getting different treatment and I predict that in the near future, before you know it, he'll have decided - for some loony and selfish reason - that he won't be disciplining any of them. Which is crazy and so so disrespectful to you if the skids' bad behaviour is affecting you and your son as it sends them a message that they can treat you and your home however they like. Sorry if I'm speaking out of turn but you only get one life and every day you stay is another one where you're unhappy. Maybe if you left he might finally get his act together ? .... perhaps then and only then he'd realise what he was throwing away and belatedly make an effort to change ? But really, it sounds as if he's made his mind up once and for all and yes, I totally get why you see him as weak. It's totally unedifying for a grown man to be so terribly controlled by children and insulting, as his current partner, to watch him kow-towing to his ex like he does.