If he is used to cosleeping do you think he might stay asleep longer if you lay down with him at first, or resettle if you lay with him for a few minutes? I know it's hard with your own kids there too, but this reminds me of when I used to look after my neighbour's little girl. She was a bit older, like 10 months I think, when I started taking care of her and her naps were terrible too. My DD1 was 15 months-ish at the time, and I used to set her up with a tv show and a snack while I lay with her friend. At first it was hard because DD1 wasn't always happy to just be sat by herself, and the other girl didn't know me at first so could take a while to settle. But it got better quickly as she got used to me she would go to sleep much quicker and stay asleep, and DD1 liked saying goodnight to her and knew I'd be back soon. I looked after that little girl for about 18 months until we moved away, and by then I could just put her down in my bed and leave and she would take a 2hr nap! Her mum was like ??!! as she still had to lie down with her and usually got a 45min nap! But you know how kids can act differently for their parents, so maybe as this little boy gets older it will work to your advantage that you are a different person and place than his usual sleep environment. I don't know if it's feasible for you to lie with him for a bit, but just wanted to share what worked for me. I have never done PUPD either so I'm a bit hazy on the specifics of that, sorry.