There are ways you can do it with deniability built in. Switch off your phone. Then be "visiting family for dinner, out of cell range". Forget your phone under the cushion on the couch. Your nephew was playing loud music and you didn't hear it ring.
If the boss says that they need you available for emergencies or whatever, say that your therapist suggested you disconnect in the evenings or you will burn out irreparably, etc.
Have a talk with your boss and discuss the current situation and how it is not managable. Work out a solution in the short term. What happens next is that your boss will totally not stick up to his end of the agreement, because he will assume you will fix it anyways. But then you stick to yours and let things burn. Then he gets the hard lesson, and you can put the fault in his shoes since it was him who didn't do his part. After that, things should be clear who does what.
In my experience this works, and my empoyers were always very happy to work with me. You don't need to be a pushover to get respect, you just need to do a good professional job. Expecting your employees to be available 24/7 is not profesional, teach him that.
Thanks, I've made a note on my phone which reads 'Let things burn', to review frequently. My employer refuses to set up an on-call rotation, probably to save money, but they will and frequently do call you 24/7 expect you to work immediately, for no additional compensation. This after a week of constant task switching plus a 3 hour total daily commute is just too much to sustain.
I still need to figure out how to deal with the frequent 'can you work next Saturday and Sunday?' with a thank you as payment.
Working in the weekend without pay seems like a problem of your employer and not you. By asking "can you work in the weekend because blabla" he puts that problem onto you. The trick is to point the problem back to him. For example you can say "seems like you need to hire another person, is that planned already?". Or "You need to put a better planning in order" etc.
Try to get concrete plans and dates out of him, and then use that as excuse to not do things "we agreed you would hire someone else at x, you didn't do that, and now I have plans for the weekend. Better make work of that soon!"
He will always try to find excuses, but try to steer the problem to him. Because in the end, it is a failing on his side that you need to work in the weekend, make it very clear it's his fuckup, and if he doesn't fix it soon, it will be him in shit and not you anymore.
If the boss says that they need you available for emergencies or whatever, say that your therapist suggested you disconnect in the evenings or you will burn out irreparably, etc.