When I moved in with DP, his house was exactly the same. It was such a shame because I never felt that I could ask my family round for a brew etc as I was just embarrassed about it. Over time, we've gradually addressed the issue, but it's not something that's fixed overnight.
I told him I would help him (he seemed to want to make a difference to it, but it had gone so far it was now overwhelming) but that when each part had been tidied, it had to be kept tidy.
So, we tackled it room by room, including the garden, and sometimes it took us months to get a room done as we're busy people, so often it was just a case of 'we'll tackle this corner today, that pile tomorrow' etc.
Lockdown allowed us to get a huge amount done, our bedroom is completely clear and there's now NOTHING that isn't put away. Nothing on the floor. It's amazing. We did a lot in the living room too, but that's still messy as we need more storage and places to put things we do actually need. That's getting decorated with new furniture in the new year so that'll be fine.
We got the kitchen done, we ripped it out ourselves and had a huge clearout along the way. Garden is clear, DP's son's room is as clear as a teenage room can probably get.
We have an office room that's absolutely full of crap but that's kind of being used as a dumping ground while we get things done to create more storage, so we've not attempted that yet - will do that last when the rest of the house is decorated etc.
But the point is, I didn't just leave him to it. I helped him address the issue, under the promise that it would never get that bad again. And it hasn't.
The important thing is, though, that he has to WANT to change. If he's not on board, or he doesn't see it as a problem, you're just going to struggle forever. I promise you that.