Hi there, just wanted to jump in and give you an extra opinion:)
I wonder if the switch to 2 naps was too sudden for him. Sometimes even if you see it's 3-2 time, it can take up to 3 weeks to make it adjustable for LOs. What I mean is, even if you know 3 naps don't work anymore, I would not cut the 3rd nap in 1 day. Does that make sense? If you do it quickly, they still tend to take their time to adjust. The best way to handle it is to be persistent with timings I think, for a few days.
I agree, that the older they get, they seam to react better to constant nap timings i.e. my 15mo DS rarely gives me any tiredness signs, but I just start routine at 1 and he falls asleep whenever. It takes 5 or 30 min. He still chooses his time, but has quite time at a certain hour. Does that make sense?
About the routine... From what you've written you've just transitioned to 2 naps, so I wouldn't think of short/long naps for at least next 3 months. Right now, he just had to cut 1 nap, so I would let him have his 2 full naps (1.5-2h).
I think spreading these A times equally could help. If he can do 4h afternoon, he can do the same after good night sleep, but you have to get there gradually:)
I would suggest something like this next 3 days:
7:00 WU
10:00 nap 1, (start the routine at 10, he will probably WU around 11:00)
2:00 nap2 (if he wakes at 11 do 2pm, if 11:30 2:30)- try not to go over 4pm
7 BT I really don't think he will be UT for that BT.
What do you think?
It will probably fail the first day, but day 2 or 3 should give you some peace