I wonder if he thinks that if you weren't around to pick up the childcare and support, then his parents would be forced to get back together
I agree. Your DSS is actually pretty astute. In the nicest way, his logic is that if you weren't in the picture then his parents would have to see him more.
It certainly sounds like you are doing way too much for him when his parents should be stepping up. The narrative on MN is always 'treat your step children like your own and love them like your own' - because that makes the adults in the dynamic feel better about themselves. But here we have a good example of how that's often not what the child wants.
The child wants to be parented by his parents. It would be much healthier if the step mum could take a step back and try and facilitate that to happen. Rather than become the default main parental figure. It's confusing and hurtful for the child and there are no boundaries for the step mum.
This is so typical of what happens to step mums. They're treated as glorified nannies and are expected to be the person who takes responsibility for fixing the broken family unit and making it all better again. Even though (unless they were the OW) it's nothing to do with them and all to do with the poor choices of the parents in the first place.
I am a step mum myself and see it in my own family dynamic. It should have been glaringly obvious to anyone with half a brain that it was a bad idea for my husband and his ex to have a baby, given their circumstances at the time. Yet they went ahead and only two years later, surprise surprise, they split up.
When I came along, I quickly felt the pressure from all sides to step into the 'mum' role and make it all better. Naive as I was at the time, I accepted it as my duty. (MN was not helpful here; every time I posted about how I was feeling, I got torn to shreds for daring to suggest that I wasn't comfortable being cast as a third parent.)
It took me ten years and one horrific bout of post natal depression with my second child to find the strength to say enough. I'm not my step child's mother, nor do I want to be, and most importantly, nor does my step child want me to be. It was convenient for my DH and his ex, that's all. By having me to pick up their slack, neither of them had to confront their own guilt about not being around enough for their child whom they were letting down. It was not my job to shield them from that, but when I pointed it out, they didn't like it!
So I put some cast iron boundaries in place. It ruffled some feathers at first, but I stood firm and everything is now much better - including my relationship with my step child. Not least because I am no longer a seething ball of resentment, stretched to my absolute limit caring for three children while their parents enjoy the advantages of my unpaid labour.
Your DSS is doing you a favour here, OP, he is actually making your argument for you. The question is what will your DH do about it? Because if the answer is nothing, then you will find yourself faced with that old MN classic - 'A DH Problem'.