Hi Shiv - thanks so much for your reply.
We were doing short AM/long PM to let him catch up on his OT yes. Yesterday I wasn't sure he had fully caught up (even though the signs were there

as he took 20mins to settle for all naps the day before), so I carried on. Clearly the A times we are using are too short.
I also agree that capping the PM nap doesn't help. All it did was make him OT. I tried to argue a case for making BT later, but DH doesn't agree. He thinks stretching the day is worse than not giving him enough A before bed whereas I disagree with that, as insufficient A will just make him EW, and cutting the nap means he hasn't had enough sleep.
DH & I talked & we both agree that my plan to do 4hrs A long nap & PM catnap won't work b/c he just won't take the PM c/n. And 4hrs A in the AM is just not enough to get him through the day, esp when we are starting with an EW. For example, if I go back to 4hrs A today, that is only 9.45am. Even with a nice 2hr nap, he will be awake before midday & so we are looking at 6hrs+ A time to BT if he refuses the CN, which will just make him more OT. So how do we get round this

I know some OT is inevitable in the transition, but you have seen the effect it has on O. Just a tiny bit of it & we get EW virtually straight away. Then it makes it so very difficult for us to carry on.
I think maybe we need to do a longer first A than 4hrs, more like 4.5-4.75hrs & maybe we have a fighting chance of a decent nap & getting through the day. My biggest problem ATM is that DH really doesn't agree. He thinks 1 nap just makes him OT, and yes I agree it does, if we keep allowing stupidly long days like 13+hrs. So I feel like I am stuck trying to muddle through with 2 naps.
Siigh. I'm off today since he's not allowed to nursery, so I actually have a full week where I could implement a decent routine. Do I just go for it?
ETA: I am an idiot & I have chickened out again.

He started rubbing his eyes at 9.15am despite changing activities & then at 9.35 got really fussy & yawned so I PD (we hardly ever get yawns so def a good tired sign). He fussed/cried for a good 10mins, not at all like yesterday when he was chatty & playing in bed & most definitely UT. I think he is already OT & I'm not surprised after the mess we got in with his naps yesterday. Asleep 9.50am, same as yesterday. 4hrs 5mins A. It looks like I have no choice but to go with the short AM/long PM again today though. I can't risk this being the only nap of the day can I

So I will do 30mins AM nap, 1.5hr PM nap & we will take your advice & stretch BT if needed, Shiv.