Thanks for the input ladies.
KDee - Such a deliberation to cap the nap at 1.5hrs. It frightens me more than EBT! It would still put the catnap at 5/5.15pm - am scared it would be too late & he would just refuse. And if I attempt & he refuses then it will end up pushing BT later by the time we have got him up & got him ready for bed.
Vicki - I agree if I let him sleep & he does a 2hr+ nap really the only option is 1 nap. This is what happened the other day when MIL had him, but I think we did BT too late & we got EW the next day so I'd have to be REALLY careful with this.
WRT a shorter A to BT - I don't think its a problem if he's already done 2x 4-4.5hr A times in the day. But TBH I think that at this stage its just making his day too long. He is awake for 12hrs every day - he is going to get exhausted & this could harm our nights as much as anything else. We've been here before & 14hr days are just not sustainable long term.
My biggest worry this AM (Vicki - WRT thinking of the long night as him getting used to the new routine, rather than 'catch up' ) is that he is already yawning, only 2.5hrs after WU. I fear pushing til 11am is going to result in an OT nap.

I am going to stay with it but think I will probably go in the middle with 4.15 A time, unless he seems like he can go the 4.5hrs.
I'll update tonight with how it went.x
ETA: Shiv - just saw your post. 4.15 is what we'll do. Hmm still undecided on the capping of the nap but we'll see.