Hmm... the other way around it is to hold on to the routine you have now for as long as possible so he can have a chance to get older and more able to cope with the 1pm single nap day. The routine now appears he is self regulating his sleep by sometimes waking earlier and sometimes later and with the little CN at a set time. This looks good although not necessarily nice for you to have the uncertain wake up time.
When he is older you could then move his day on as much as you can with a later BT, later morning CN and then on a day when he wakes later (rather than earlier) in the morning you take the leap to one nap. With the morning WU as late as you can get it (obviously you need to get out to nursery etc) you could then drop than little CN and move to the 1pm nap. You might find he just nods off in the car seat at 12.30 pick up and you might need to transfer to the cot at home, mine did eventually agree to being transferred from car to house mid nap. I would begin talking to him before touching him and keep up the chat the whole way inside and up the stairs (I used a sling as I had a fair walk from the car then 3 flights to climb to get to his bedroom), beginning with "I'm going to pick you up now, stay asleep, everything is okay, it's just mummy, I'm taking you to bed" continuing with "okay okay, I'm taking you to bed, stay asleep" and when we got there "I'm going to put you in your bed now so you can sleep properly, everything is okay, sleepy time, night night, call if you need me" . He half stirred a few times, nuzzling on my chest in the sling and then would snuggle into his bed so I know it wasn't a deep sleep but he did finish a 2hr nap this way.
Sorry not to be of more help for you!