Hi there
One the NND (no nap days) are you putting him to bed earlier? the usual advice is if there is a NND then to bring BT super early, even 5pm! It doesn't always result in earlier waking but if it does then the next day you might find it easier to get him to take a nap, even a capped nap.
Many people get through this phase with either a mixture of nap and NNDs which balance out reasonably well to keep things kind of predictable. Or they do a nap every day but much shorter until the final nap drop.
Then there are the kids who nap every day until suddenly they don't. Mine had to have 2hr nap daily and couldn't last without it, I tried capping with awful result, he was better on a long nap and shorter night and i just accepted late BT and earlier waking with the shorter night because he was in a great mood so the whole day was easier as he wasn't such a horrible grouch. Then he fell ill and during his illness napped 2.5hrs, I let him because after all he was ill. Well, that was his last nap. No more naps. Nodded off the odd time in the car for a few mins here and there but he was done with naps. The good news was that he was then able to do 12hr nights which he had never done before and the 12hr night lasted a good couple of years, he was about 5yo before his night length shortened a bit.
Sorry, no solution, just a couple of ideas.