So if DH wants to be able to do bath time, he needs to be able to set his own stress and tiredness aside so it’s a relaxed, enjoyable experience for them both.
I’d have an honest look at how you both parent, and find a middle ground you can both agree on. In the early days I’d say my DH was more the disciplinarian and I was more laid back. In reality I was letting things slide because I was exhausted and just didn’t want the challenge - my kids favoured me because I didn’t challenge them. We needed to agree on what was/wasn’t ok, and recognise when one of us was reaching our limit and tag team.
That’s only possible if you know between you what your standards are. You also need to agree on what boundary setting and discipline looks like - my DHs household was more consultative whereas my parents were very authoritarian and my parenting style pushed back against that. If you’re waiting until you/DH is cross before setting boundaries, you’re waiting too long. The trick is to have standards that are consistent regardless of of your personal annoyance and reinforce them consistently. And to know when to walk away. It’s not that you or DH can never be cross, but if you wait until a behaviour is annoying you, you have to deal with your own emotion and try to regulate your child, which is a much harder task.
Revisit discussions about how you wanted to parent in your assessment and think about how that’s turning out in reality.
While you say it’s a problem of DHs making, and there may be some truth in that, you parent the same child, so you have skin in the game. It’s in your interests to find a balance and create space in your relationship with your child for dad to step in.
I’d also be really exploring the “monster” comment with your child, finding out why they’re using those terms and nipping it in the bud. Help him find other language to express him feeling unfairly treated or unhappy with something. Calling someone a monster isn’t ok and must be hurtful to your DH, which won’t support relaxed parenting. It also sets you up against your DH in a good/bad parent kind of way when you need to be a cohesive team.
It sounds like a horrible environment for your DH, I’m not surprised the bond isn’t there. What do you do with your child, can DH join in with that, or model to him how to engage in your child’s interests? As the “preferred parent” you have the ability to ease your DHs path, think about the kind of support you would want if your DH was the favoured parent.
Its very difficult in adoption because being a parent holds a whole set of preconceptions, hopes and dreams which don’t always fit the reality of parenting a child with complex needs. My DD is older but with a similar needs profile to your DS, and we’ve had to work together parenting her. Her dad was her preferred parent and I’ve had to really work at our bond, it’s solid now but my DH and I had to work together to get there. Try to see your DHs relationship with your child as being intertwined with your relationship with your child, and with your relationship with DH. They work together to create a “system”, to change one part, you need to change the whole system, you can’t do it in isolation.