If his A times and his napping routine is "normal" for his age, why did you mention in one of your posts that he is young to be that far into transitioning?
Sorry I am not always clear in my writing, I know what I am thinking and know what I mean and dont always word it so clearly!
I don't think his napping routine is appropriate, but I think that 4h of A time is not abnormal. Certainly many are on less A time at that age, prob most, but def there are many on 4h at that age. BUT to be so far into transitioning that he is only doing a 15 minute morning nap is way young....although then reading your later posts I think I had read that wrong as you said it was just a one off thing?
I think that a 1.5 hour nap is def right for his age but yes when he is doing so little in the morning it is not long enough because then he has not got enough daysleep in him. BUT I don't think the answer is always to cut the morning nap to make the afternoon nap longer. It certainly is the right thing to do in some cases, but not all, and I think it can be a fine line with the 2-1 switch to know which is which. Going to go on his age more than anything, but also some of the things you have described as having happened in the past few days and say that cutting it is NOT the right thing to do in this case. I think that his shorter afternoon nap is not going to get longer by cutting the morning, I think that the way to get more day sleep into him is to give him the extra bit in the morning.
For offering ideas, it is hard for me to put a "routine" idea down that will work for you! I can just give some ideas of what I think is happening and you can see what you agree with and don't agree with!
One thing I would question is if he is for sure doing 4 solids hours of A time consistently, and being happy about it all the time. As in, are you positive that he is not doing 4 hours because he is just having nap refusal from being OT, and so you are assuming that he is up to 4 hours because that is how long it has taken him to settle the past few days? If so, that's fine, I just want to check to make sure! Also, how long has he been doing 4 hours? Maybe he is working his way UP to 4 hours but after a day of it, it shattered him and so he needed to go a bit lower the next day....sort of like older toddlers with days that alternate between one nap and two, they can handle the 5 hours of A time to have a one nap day, but after doing it one day it totally wipes them and so the next day they need to go to two naps/reduced A time. And this can go back & forth for a few weeks until they can consistently handle that 5 hours of A time every single day. So perhaps he is moving to 4 hours, but because he did it once, or twice or even a few times, doesn't mean that it didn't totally wipe him out and he needs a few days with lower than expected A times to catch up from it.
Does that make sense?
So no advice on WHEN nap routine changes, other than that I personally would not make the first assumption to be that he is way ahead of the game with the 2-1 and A time and jumping A times drastically. I would assume that these things are occurences but not his normal "new" routine.
I would also say that if you have him down to 15 minutes in the morning, you are probably better off doing a day with one 30 minute morning nap and a shorter afternoon nap, alternated with one day of ONE nap only, at 4.5 hours A time with an early bed. The thing is, I really do NOT think your LO is ready for that yet, but I do think that is a better option than a consistent 15 minute morning nap and 1.5 hour afternoon nap for a toddler who is at that stage of the 2-1. Just something to keep in mind.
I think that you might find some success with a morning nap of 30-45 minutes at 3 - 3.5 hours of awake time. I know your gut will be to say that he won't go down at that time, but if you keep the morning low key, send him some sleepy signals nearer that time, or do a wind down and then pop him in his stroller and see if he'll nod off if you go out for a walk. And then the day would look something more like this
wake 630
nap 945 - 1030
nap 215? - 345
bed 730
What I started finding at this age was getting too stuck on how short the morning nap would be and not looking at the FULL picture of everything - naps, As, nightsleeps, NWs, EWs, mood, development, and so on, and instead just looking ONLY at A times got me into a mess. I did not cut DS's morning nap CONSISTENTLY until he was 13 months. I first cut it down to 45 minutes when he was 9.5 months but then would have days where I just had to use my instinct and let him sleep 1 hour, or 50 minutes. And some days I got a shorter PM nap out of that because I had been wrong, but most days my instinct was right. When I was stubborn and insisted that I wake him at whatever length it was cut to, we got OT and then afternoon nap refusal which then just led me to believe he'd had a massive A time jump when in fact it was OT. Once I cut the nap to 30minutes, I had to sometimes let him go to 45 minutes, again on instinct, for the same reasons. And the same when I went to one nap - every now and again it had to go to two naps. So I think that just because you have dropped morning nap to 30 min because you were having PM nap problems doesn't mean that he can ALWAYS do 30 min in the morning without OT creeping in on him.
Not sure...is there anything in there that you find useful? Or is it all just ramblings? LOL!