This is difficult for us, as honestly I do want to say to keep pushing on through, children do eventually (when super super tired) actually sleep. I do think you could be just round the corner from it clicking and getting to him adjusting to the new pattern.
But, and this is the main problem, we aren't with you, we don't know your DS, and we can't see what you're seeing. At some point you must follow your mummy instinct, and I wouldn't want to tell you to ignore that.
Looking back at our conversations about your DS's sleep over the past months (possibly almost a year) he has never really been that well rested and perhaps my view that he actually is quite LSN and needs less (or no) day sleep is correct. Perhaps the cat nap is actually more trouble than it is worth. But I don't know.
What do you want to do?